Anyway I'm a total beginner here but I really want to grow a bonsai tree in my house so basically am I best to just get one that is allready growing and just find out the best care for it?
That would really make things easier on you. Make sure you get something that is suitable for indoor culture if that is where you want to grow. Some trees do Pretty well inside with the right care but some really are inappropriate.
Indoors, Ficus and Schefflera are good choices, of course there are others. trees that are inappropriate indoors are Junipers, Maples and most Pines. Basically look for tropical plants.
Since you don't know what type of seeds you have it would probably be safer to wait until next spring and plant them outside. Some seeds require a cold chilling period in order to break dormancy and some do not. Without knowing what type of seeds you have you are at a real disadvantage.