While many, if not most, instructional writings never really broach this idea, it really can be the most important. Do you like it? There is no question that fonts have an aesthetic quality and that people react to different fonts in different ways. At the core of choosing a font that best expresses your message is you. You know who you are and how you present yourself so let the things you like speak for you. Fonts included.
For most people fonts are like plastic forks. Stay with me here. They order take-out or fast-food and usually assume that since they are paying for the food, the utensils come for free. In that same line of thinking, people buy their computers and software and probably assume that the fonts come along with them like free utensils. I would bet that many people don't put together that the costs of their plastic forks and their fonts are baked into the price.
All metaphors aside, fonts cost money. Ok, not all of them, but 99% of the good ones do (and many of the 1% left are commissioned and then released free to the public, see Cooper Hewitt & Fira Sans). While most people won't think the cost of a font is worth the money, since so many come "free" with their computers, there are plenty of occasions where those System Fonts don't do the job you need them to do and the need to purchase a professional font comes into play.
I don't want to get into a long triage about how this system works so here are the basics. Fonts are software. You don't purchase software, you license it. License fees vary based on 3 main criteria; how many fonts you need (light, book, medium, bold and maybe some italics?), where the fonts will be used and how many licenses you need to purchase. The more people and platforms (print, websites, applications, etc.) you need the more it is going to cost.
I know I am throwing a lot out there, but I just want you to be aware of how multi-faceted this process can be. A very practical application of this knowledge is if you only have $200 in your budget to buy a font, spend some time to figure out exactly what you need and where you are going to be using it and maybe don't buy the full family if you really only need Light and Medium.
This expands upon the idea of "Do you like it." You may like 4 different fonts, or even 42, but there may only be 1 that speaks more specifically to your message. I love Clarendon, Didot, Futura and Garamond but I would be wary of using Didot for a project that needs to feel rustic and comforting. A good way to help you better figure this out is to list out descriptions about your project using words that usually do not apply to your project.
Think of it like the game Guess Who? You work through different character traits till you are left with the exact match. Defining the character traits of your project will help you find a font, picture, color or layout that has traits in common with your message or project.
These next two dive into how the font will be used. Like all software, different programs are needed to run on different systems. Your Angry Birds app on your iPad and Android phone may look and work the same, but they require different types of files built for different systems. Much in the same way, font files that work for Adobe products or for Word or Pages may not work for fonts to be viewed on a website and on top of that, different web-browsers don't even read the same font files. I keep using phrases like, "I don't want to overwhelm you," and then list a bunch of differing ideas but the first step to understanding is acknowledging. Much like licensing, different platforms have different requirements and require different things to make them work. So do you only need to make static content, like a book or poster or a PDF that is going to be distributed, or dynamic content like a website or an email that is built with code? Just think about if you need it for one or the other or both as you make your decision, and look for an option that meets your needs.
Size does matter. Some fonts are made to be big, and some are made to be small. A few are made to be both. If you are only looking for a headline font, your options shift a bit. You might also want to think about how much text you will need to fit into those headlines. A great example of a typface made for headlines is Knockout. Knockout has 9 different widths and is made specifically for headlines where you might need to fit a lot or a little text into the same size spaces, all while keeping the same size. While this is amazing for headlines, it is not meant to be used for large blocks of body copy.
Conversly, there are many fonts that look great at small sizes yet can't hold up when you blow them up. I wish I could give you a good rule of thumb but it really doesn't exist. The best thing to do is research it as best you can. Read the type specimens from where you plan to purchase the font, do an image search and see how it is used by other people or if you feel you are really in the dark, most type designers and foundries are filled with really cool and nice people who love to talk about their work. Email them or talk to them on Twitter, they are full of knowldege and some great nerdiness. Plus you get around 10,000 cool points for having a friend who is a type designer.
Alas, at the end of the road we come to the great extistential quandry of need. This is where my greatest weakness lies. I covet typefaces. I follow type designers on Twitter and Instagram and tell them when their new typeface has a particularly sexy Q, (this really happened) and would spend my life collecting beautiful fonts. The only thing keeping me from this dream-life is the need to pay bills and feed myself. This hasn't stopped me, only slowed me down. After a year or so of joyful impulse buying, I realized I was buying these beautiful pieces but probably never going to use them. I had to make a rule for myself that I would only buy new fonts when they were specifcally needed for a client project, or had a great sale! So at the end of it all ask yourself, do I need it?