I have registreted on site for LOTRO, and I still have the game discs, should I download the whole game, or can I install the main game and just install the patches?

What about all the expansions that has been made?

Downloading might be faster, the Shadow of Angmar discs are horribly outdated by now but both options will probably take a lil while, game is big.
 
You need not worry about the expansions.
Mines of Moria added two new classes but you already have all the originals to chose from. 
Them being Hunter, Champion, Guardian, Lore-Master, Minstrel, Captain and Burglar.
 
All Moria content is accessible from lvl 45 but recommend for lvl 50 and up.
Mirkwood is lvl 60 and Isengard starts at 65, so that is a fair way off for any new player.
 
And one very important tip: ENJOY the game, do not rush levels as it gains you nothing, this is a game based on the journey and the experience, not getting through it all as fast as possible like some other MMO's.
Enjoy, turn up the music and live the game.