In general, the risk of a man getting HIV from a woman during vaginal intercourse in the United States is low--probably less than 1 out of 1,000 exposures will result in actual infection. This risk may be higher if the woman is having her period, and may also be higher in poor countries.
Of course, there is no risk of getting HIV from a woman unless she has HIV, so it's good to talk about this with anyone you are thinking of having sex with. After all, she may have the same thoughts or concerns about whether YOU might have HIV, but might be afraid to bring it up too. And since it's often hard to be sure, especially if you don't know someone very well, remember that using a condom can greatly reduce the risk of spreading or getting HIV and other diseases, and can be a good way to show that you care about your partner.
An even better way might be to get to know your partner really well before having sex, which makes it easier to talk about HIV and pretty much everything else.