Have you actually purchased the game? If so, the game does a good job of filling in the blanks for you as you go. The mystery of what is going on is the best part. In this case, Black Knight's summary is about as much as you want to know for now, IMHO.
If you haven't purchased the game(s) yet...I would suggest starting out with BG1 and then proceeding into BG2...the storyline/plot is more linear that way.
You should expect about 300 hours of gaming time out of BG2 and not much less out of BG1.
Welcome and enjoy!
P.S. - Black Knight did not mention that this is a party-based game. You play one character and get to add people that you meet (NPCs) to your party. Some of the best parts in the game (IMHO) revolve around interaction with NPCs.
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Now where did I leave that doughnut?!