Help please!!! ques about HIV?

Ok i have heard so many different things about this but how hard is it to catch HIV of an infected person if you have unprotected l (im a boy)...people have said its very hard even if she has it if your male and you have only done it once...ive seen number likes 1000/1 which is hard to believe but any1 with some knowledge about this if they could give me an answer it would be great. Also if there is no sores on at the time.