Someone on twitter has "my" name?

[Liana]

Chances are, you can't. Even if the account has been deleted, I think they still won't let you use that username. The closest to the username you want is probably changing some letters (for example instead of "a" you could put @, and instead of "i" you could say 1 etc.) or adding a number to the end, or adding _ between some letters. Also, if you follow them, it's possible they will follow you back so they can see if they know you but if it's an unused account then that person will never go onto the account to see you're following them.