Just curious. As there isnt anyone online yet for me to join, so I am playing solo. So, when people do eventually get online, will my main characters from my solo game be transferable to online if i join someone in a co-op game? Or do i have to start from scratch?

My guess would be that you would have to start from scratch if doing co-op would start a new game

guessing-time:
well, the videos look like this: you play the campaign and gather items, gold, etc.
then you can set any singleplayer campaign to a multiplayer one, starting from where you were in the last savegame, still having all the items.
and then, if you go over to play a Crucible map, the only thing that stays is your money to unlock entities for your selfmade dungeons and items
you can equip the players with at spawnpoint.

so, it's not told that you will start from scratch for new campaigns, but it kinda looks like it.
if you start from scratch, i think, the pre-order items will stay in the inventory, so if you've pre-ordered you're not completely on the lowest equipment level possible.

that's just my subjective view of what i've heared in trailers.
the actual answer will only be attained at release next week or via an official gamesas post.