These Help pages can be downloaded here with the Winning Bridge Shareware game for Microsoft Windows 95+.
If you would rather read a book, take a look at the Help Index page for some suggestions.
Overcalls
When the opposition have opened and you might want to bid if you have a good hand or a weak hand that is worth a sacrifice bid.
With 16 - 18 points, a balanced hand and control over any suits bid by the opposition you could bid 1 no trump.
If you are using the take-out double option you could use that.
If you can make your normal opening bid at its normal level and only one opponent has bid, make that bid (except do not try a weak no trump opening).
If you have a hand that is suitable for a preemptive bid and the opposition bidding is below the level of you potential bid, then you could bid the preempt.