Teriyaki is an MFA in film directing with several decades of professional experience in media production.
First, a secret: good video is 90% about sound
When you are watching a movie or a TV show, what is most likely to break your immersion, enjoyment and what we in the trade would call "willing suspension of disbelief?"
The answer is quite simple: bad sound. It seems that human brain treats the two senses used in perception of video content quite differently - while visual information takes a lion's share of brain's "processing power", perception is much more sensitive to sound, particularly any imperfections and sudden deviations. The reason for this may be evolutionary, with our ancestors relying on sound cues to detect danger much more heavily than on visual ones. And of course, unlike with vision, you can hear what's behind your back..
So, the result of this is that your viewers (and listeners) will react much more strongly to any imperfections in sound than to those in video. As a matter of fact, imperfect and glitchy video can present a pleasing aesthetic on its own. Imperfect and glitchy sound on the other hand is ALWAYS perceived as irritating and distracting and thus to be avoided (unless you are working on a horror movie, but that just proves the point).
Get yourself a better microphone, duh!
But here's the good news! Achieving good, if not top-notch sound for your youtube clips is much easier and much, MUCH cheaper than accomplishing the same with video. An almost-professional grade microphone can be bought really cheaply if you know what to look for while a video camera of comparable quality can set you back for almost the price of a new car...
So, the conclusion is quite obvious: Get yourself a good microphone. Whether you'll be using it outside in the field or in your home studio, a good microphone will easily act as a real value-multiplier in anything you do involving real-world voices and sounds.
But what kind of a microphone would be the best for you? Well it depends... mostly on what type of sound you plan to record with it. Read on, gentle reader...
What kind of microphone should I buy?
The type of microphone you'll want to buy depends on the kind of content you produce and where you are producing it. With that in mind, the most important characteristics of a microphone are its polar pattern (how sensitive it is to sounds coming from different directions) and its ergonomics - how you physically use it, the size of it, where it can be mounted etc.
In the case of a typical talking head vlog (or podcast, for that matter), where you alone talk to your audience in an enclosed space, a unidirectional (or cardioid) studio microphone is the one you should be looking for. "Cardioid" means "heart-shaped" and describes the pattern of a microphone's sensitivity to the sound space around it - whatever is in front is the loudest, but it also does register a bit of ambient sound from the sides. This is ideal if you want your speech to be loud and clear yet not completely disembodied. Subconsciously, your audience will perceive you as a more "real" person if your sound includes some ambience of the space you physically reside in. In short, it gives you more "realness" - quite an important factor in videos where you want to engage and capture your audience on a personal level.
However, if you do have a problem with ambient noise, say you work in a noisy office or crowded household you may want to get a "shotgun" mic instead. These microphones are unidirecional as well in that they are more sensitive to the front, but their pattern is much tighter than is the case with cartioid mikes. These microphones are a bit more difficult to work with in a desk environment because you have to be extra-careful to keep the mike pointed directly at your mouth and throat area, but they usually manage to isolate what is important from the surrounding soundscape. Shotgun mikes can also be used when you go out and, for example, want to conduct an interview - most of them have standard hotshoe mounts and can be coupled with a camera to give you an acceptable level of sound... but just remember to keep your subject close to the center of the frame!
The third type of microphone is the Lavalier. Its unusual name comes from a french duchess by the name of de la Valliere who used to be a mistress of King Louis XIV of France (also known as the Sun King, /rolleyes...) and who, among her many accomplishments, is credited with inventing and popularizing a new type of neck jewelry which quickly adopted her name. Roads to popular immortality are mysterious and tortuous indeed... But enough of interesting historical distractions, Lavalier mikes are those little black things you see dangling discretely from lapels of field news anchors, documentary hosts etc. These tiny microphones are invaluable if you plan to move a lot, but they can be fiddly to use and tend to require a special sound monitoring person to check if, for example, it is brushing against clothing, pointing the wrong way etc. The more expensive ones can produce output of surprisingly good quality and are a very viable solution if you plan on shooting outside or are making an instructional video requiring you to change positions often, especially when coupled with a remote receiver to get rid of the wire. For most "desk-jockey" types of videos they may be a bit of an overkill though.
The final decision to be made in choosing the right microphone is whether it is working on condenser or dynamic operating principle. While shotgun and lavalier mikes are almost invariably of condenser type, desk and studio microphones can be either of dynamic or condenser type. So what's the difference? In short, condensers require to be powered separately, either with batteries or by using an outside power source (known in the biz as "phantom power"). This makes condenser microphones more fiddly to set up and use but they can give much better sound at lower cost brackets. Dynamic microphones use energy of the sound itself to power themselves and are thus generally more rugged and less sensitive - however, a really well made (and thus expensive) dynamic microphone can easily rival condensers which is why they are favored by rock stars for their live performances. They can take a lot of punishment but when they do break it's scandalously expensive, perfect!
I've decided on the mike type, what next?
In case of some microphones, that's it! Just take care you don't clip your sounds (never go red on the vu-meter, NEVER). If you want to accomplish this like a pro, you'll do it by getting yourself a dedicated piece of equipment called a "mixer" which... mixes sounds! It's got all the knobs and dials you might never need and allows you to adjust the microphone's analog output before it gets converted to digital signal in your PC - which is a good thing, trust me. And better ones (read "more expensive") can even provide your condenser microphone with phantom power which is great if you want to keep the desk clutter down.
Second, some cheap accessories can boost your sound quality immeasurably. Go and splurge on a foam cap, shock mount, pop filter and a phantom power supply if you require it (I hope you've learned whether you do by now). All these accessories can be got for almost next to nothing and are often bundled with the microphone. Don't skimp on those - a few bucks for a piece of plastic mesh can make your habitual hisses, spits and sputters that much more endearing...
And finally, take some time to learn how to master your sound once you got it in the PC. Play around with audio filters in your editing software (start with the equilizer) and make it your rule never to release raw sound from the mic onto an unsuspecting public. It is amazing how much screen dollars (or speaker dollars) you can earn by simply putting just a little more effort into the finished product.
Oh, and one last thing...
Get yourself a good pair of headphones. Listen to the sound in the best possible conditions, critically assessing any subtle nuance and imperfection.... Once you're satisfied with the results put the headphones gently away. Now listen to that same sound on the most horrible speakers you can find, say those in your crappy laptop. Remember, most of your listeners will probably experience your creation on equipment that is somewhere between the two extremes and if you master the sound for one extreme only, the chances are the other one will turn out to be sub-optimal. Cover all your bases!