I remember vaguely some of the stats when I was doing my own research on condom effectiveness. I know you can find all of this information at Planned Parenthood. Something like condoms alone have a 14% fail rate. Spermicidal lubricant alone has a 23% fail rate. Birth control has a 1% fail rate. Birth control with condoms is one of the best options but it isn't for everyone. Using a condom along with spermicidal lubricant is also very effective, something like a 3% fail rate.
Like a few of these responders have said, NEVER EVER EVER use 2 condoms. Slippage and tearing are much more common in this case. Also, this may sound embarrassing, but go through and find a couple youtube videos on how to properly put a condom on your man (you can never be too prepared).
Figure it all out now so neither of you are tweakin' after the deed is done.
