Go to
http://drop.sc to upload them, replays are located somewhere like...
My Documents\Starcraft II\<follow shortcut for your account name>\Replays\Multiplayer
Once you upload it a link to the game will pop up in the lower left, which you can paste here.
There's also an option you prob want to set in-game - by default it will only keep the last 20 or so replays, there's no real reason why you shouldn't want to save all of them forever.