IMO, there isn't a whole lot of solo leveling to be done with a thief or most classes unless it is low level mobs or just really slow going. That being said, there is a whole lot of this world for you to explore. Talking with others is huge too, asking questions anything that you don't understand, want advice on etc. Groups are a little tough to come by sometimes but don't let your final goal be to hit 51, rather enjoy all the different interactions and things you learn along the way there.

Back on the original subject of solo leveling. This is likely best done with good tanking classes, Warriors, Rangers and Assassins are likely the easiest to accomplish this with. Some Paladins can do this also but need empowerment. Other people will have different opinions than mine though. Taking this into account, Welcome to Carrion Fields.