Make friends; join a linkshell. You'll have a hard time finding a party as a thief untill higher levels because you can't take or deal much damage. Having a friend will let you have a party despite not having much to offer in return. Your parties will pick up around level 15, they say, so that's something to look forward to. If you know a whm friend, you can ask to be powerlevelled - entailing the whm healing you while not in a pt with you, so that you can solo enemies way above your range.