Just draw 2 vertical lines and 2 vertical lines dividing the viewfinder into 3 equal horizontal & 3 vertical portions using your imagination, and there you have your 3rds and the 4 cross-sections of the 3rds.
Well, you can try to look for books in the library. Most self-taught photography book will have the information on composition. These composition rules originates from painters who does canvas in the good old days. I don't think it hinder creativity, it does give you a rough guide. These rules are just some things which people come up that give them a good result. You can always break them if you want to, but I advise you should at least know them first. :cheers: