What difficulty level did you choose for your first game? If you're new to RPGs, it might be wise to start on the first or second level. The default, I think, is third.
To trade with gold, just click on the balance scales on your side, after you've chosen something from the trader. To get the trader to buy something from you with gold, select the item you want to trade, and click the balance scales on his side.
As for spells: those I've found in Act I are pretty poor. Focus your stat points on agility and survival, with maybe a point or two per level in something else--strength, or intelligence. You have to get very good at attack accuracy and evasion first, though, and those qualities come from agility and survival.
A spoilers follows:
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The tooth the imp wants is behind a door far to the east. In a large room where you'll find "Elven bookshelves," you'll see a door towards the south end on the western wall. To open it, turn off the nearest torch.
Good luck! <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
