Try everyone, you will at some point anyway, then find which character exaggerates your own play style’s strengths and has tools that minimize your weaknesses, most of the time if they do those two things they end up being really fun for your style too.
I usualy go for either the one with a weird playstyle (eg.Yoshimitsu), one that feels most natural to play as for me, the most elegant looking ones (eg.Raphael, Vega or Amy) or the loli.
Combine one or more of those criteria with not being overplayed and I’ve found a main.
Well basically you start out picking Viper…then you realize you should pick Viper. 
I like a character that is really gimmicky, sometimes includes command grabs or good grab set ups.
Usually I’ll play whoever i feel is easy and technical.
Learn all the characters, then pick your favorite.
It has to pleasant looking most of all, and it has to be a male. Usually this leads me to the homo characters, but that’s fine
I just keep picking random until I do something cool with someone and try them out for a while. If I’m satisfied I’ll stick with them, if not I’ll keep trying.
I still can’t seem to do this correctly for SF3 though haha
I almost always start off with a female as well, because for me it’s just fun to beat everyone using a girl!
I pick a character based on look combined with backstory, I also prefer bad characters/villains to heroes, comfort of moves usually come second to this.
As with Bison, Cammy(Alpha series made me love her), Akuma, Sol Badguy(Not bad, but character and name is awesome), Jedah, Meta Knight, Yamazaki, Ash Crimson, Iori.
I like the evil badasses, as well as the more twisted characters.
I choose a character no one else plays. In SF3 I’m the only person I know who played Q and Virtua Fighter I play Jeffry.
I pick whichever character seems like they go to church regularly, and for a second, one who seems the most concerned about the environment.
Geez 60% of you guys are missing the point…I just wanted to know the way YOU choose your main/favorite whatever you want to call it. I didn’t ask about a right or wrong way, if you should play everyone and pick 8 different people, or since its your first time playing you shouldn’t even be thinking about someone you want to main (which you should >.> I wouldn’t want to go through the game knowing 2 combos for each person when I can learn 15 for 1 person and they are all useful and deadly)
But keep them coming because some of you understand what Im saying here.
I typically pick a character that’s messed up in the head.
Whenever I pick a main I pick him/her based on their overall design. Lets take SCIV for example. First I tried out Sophitia because she was the character I mained in III and II. But she was to easy to play with IV so I started to look for other characters.
Afterwards I tried out Setsuka- hot badass character with the classic revenge story made her an instant favorite. But she was too hard for me to use at the time, so I decided to find a secondary main. I was immediately attracted to Tira, her looks, backstory, playstyle, everything about her was badass. So basically I just pick the character I think would be fun to play but have some challenges to them ,too.
after playing fighting games for years i think this is the best way to chose a main. I’ve asked all kinds of people from scrubs to champs and it eventually comes down to this. Naturally, most people gravitate towards characters that look cool but more importantly choose one based on style and determine how you want to play that character. You should spend some time with ALL the useable* characters in the cast, maybe 1-2 hours, even if you dont like them just to see how they play.
*useable - in games with large amount of characters and incredible balance issues (read: MVC2) don’t even bother fucking with the worst characters in the game. luckily, all characters in SF4 are useable.
Sagat.
No-one plays him.
Random until a certain character’s play style appeals to me.
And I never pick a girl to main.
Personally when I go about finding a main in any game I start by playing in training mode, or messing around in arcade for a decent chunk of time. I start with the cool looking ones, then go through the rest. Then I decide based on how the characters speed to damage is. Sometimes it’s just a marriage of sorts, when you play them they match exactly how you want to play(I.E May Lee is my perfect character) SF4 is by far the hardest one for me to find a main in though… currently playing Viper based on speed to stun.
Pick the shoto 
I just pick someone that i personally like, and try to sharpen my skills with him/her, and make the character the best it can be. Even if said character is said to suck or be low-tier.
Ex: Dan and Guile(charlie outfit) in SFIV.