Yes, you need to pick your favorite anime. Choose your most favorite anime in the whole wide world! It can be a really old one or a new one, regardless, there will be somebody who says ‘Hey, I know where you’re from!’.
And then pick a character you like, even your favorite. While some people don’t like to cosplay their favorite character, I personally love it! There’s nothing more exciting to me than to be able to cosplay my all time favorite at a convention and have someone tell me that I look the way they imagine the character to look in real life.
You can just start with a simple cosplay. While a more intricate cosplay is always nice, those are best saved for a second or third cosplay, once you have a little more experience. Try shopping locally and piecing together a cosplay out of parts you can find at a thrift store or things you already have. Try cosplays like Misty or Ash from Pokemon, Finn or Fionna from Adventure Time, or L or Misa Amane from Death Note!
If you or someone you know sews, try asking them or sewing yourself a simple costume out of a store bought or printed out pattern. You can also try adding simple little details like lace or sequins.
Invest in a wig that matches your character’s hair but if your hair matches the color or length, you can use your own hair for your first cosplay by styling it or using a temporary hair dye if you want to match an odd color (and if your parents say it’s okay). If anything, a wig would work better because you wouldn’t have to mess with your own hair and they are usually fairly cheap online.