The best thing I have found is just to search on the level you are after. There is always someone who has done a you tube video of it or offering advice etc. So far I have only got stuck on one and that was trying to complete the hard award.
Otherwise ask here and anyone can help you that is past that level.
As for Multiplayer... watch replays of games, play against the AI and learn their tricks, find friends who are good and learn from them.
If you're new to RTS I'd suggest playing Terran first, they are the most forgiving race.
Youtube is the best place to start watching / researching.
There are guides out there but the high quality ones require you to pay for them.
Basically, like Debedis said just keep watching replays and commentaries on Youtube.
