I've seen a few strategies for names-- honestly, naming yourself is a lot like naming a child. You can masculinize your birth name, if you like it OK, or find a male name with the same sound or meaning. You can focus on something that'll keep your initials the same, if that'll matter. You can find out what your parents "would" have named you if you'd been assigned male at birth. You can follow the same logic your parents used to name you, and pick the same "kind" of name-- family names, popular names, etc. You can look at what's more and less common for folks your age, from your area, and either pick something that will be "normal" or something that'll make you stand out from the crowd. You can base a name on something really meaningful to you, like a culture, an ideal, a hobby, a religion, or a book. You can buy a book of baby names and randomly open it to a page, pick the first thing you like. You can do a bunch of different methods, and see if anything comes up more than once, or really jumps out at you.
If you're really having a hard time deciding, see if some of your friends will help you "try on" names, by using one for a few days or a couple weeks. That'll help you know if it'll work, or be something that grates on your nerves. But, also, try not to stress too much-- nobody gets any choice in their first name, and most people live with it OK.
For me, when I realized I was going to have to transition, the name was already there (not my username, that's a pun on my family name). I looked at a few others, but knew which one was right. I went with a traditional guy name with a gender-neutral nickname, not too many religious connotations, about the same level of popularity in my birth year as my original name. My birth name is a family name, but there's an unfortunate lack of decent guys with decent names on either side of my family, so even my parents agree that's not an option.
