You've gotten a lot of great advice here. I'll try to throw out a few bits and pieces to add to it.
1) What's your skill level? If you consider yourself a beginner, you should pick mains who are easier to play and who emphasize the fundamentals--because at first you need to learn the game more than any particular character. So look for good walk speed and dash distance; strong strings that advance you forward, ideally with a mix of starters (mid, low, overhead), with some safe options; simple but effective projectiles; and good but not overly complex combos.
Avoid characters who do most of their damage through trickier set-ups or traps, have high execution demands, or an otherwise highly specialized playstyle. You can move on to them later, but at first you want someone well-balanced. Of course if you're already at a high skill level, pick whomever you want.
2) What's your preferred playstyle? MK11 looks like it's moving away from well-defined archetypes and more towards hybrid or balanced characters, but it's still worth thinking about this. Do you want to mostly be right in the opponent's face, rushing in and using speedy, short-range mix-ups and grabs? Do you prefer someone with good reach, focused on controlling mid-range space? Do you want to be even farther away and use mostly zoning to harass your opponent and shut down their offense? Pick someone with a strong toolkit for the playstyle you're most interested in.
3) Go for a balanced pair. Don't split your attention too much between a lot of characters (especially if you're a beginner), but aim for at least two, maybe three, characters to develop skill with. Every character will have some bad match-ups, so with only one character, you're at risk for counterpicking. A good rule of thumb is to pick your primary "main", figure out who their worst MUs are, and pick one of those for your secondary. That way you'll naturally have a more balanced pair with different strengths and weaknesses.
4) Make sure you like them. Been said a lot, but worth saying again: Once you've accounted for all the above, pick based on aesthetics: their looks, their animation, their voice acting, etc. You have to really like spending time with your characters in order to keep up the motivation you need to log the practice hours and level up. If you don't like them (no matter how high-tier they are), you'll lose interest and won't put in the time.
Good luck!
