I originally wrote a post that was kind of snarky and condescending here. I regret that, because it's gotten a lot of Google traffic, and I really shouldn't be a jerk to people who are trying to learn.
So the short version is:
Pregnancy can only happen when sperm contacts the egg in the fallopian tubes or the uterus. The only part of the body that connects these organs to the outside world is the vagina. There is no pathway from the mouth or stomach to the fallopian tubes; any sperm there will just get destroyed by stomach acid. So there is absolutely zero chance that sperm that didn't touch you in or around your vagina could get you pregnant.
(There is a very, very slight chance of sperm from anal sex getting in your vagina. But you really should be using condoms for anal sex anyway because buttsex has the highest risk for transmitting diseases, and even if neither of you has a disease, your partner can get an infection from getting your butt-germs in and on his penis.)
There is a possibility of getting STDs by giving blowjobs and swallowing cum, and using a condom for blowjobs will protect you from this. (Get unlubricated, if you get the chance. The lube tastes a little funky. Still less funky than cum, though.)
It's also okay to say "no" to giving a blowjob, even though it can't get you pregnant, if you simply don't want to give one. Any guy who gives you a hard time over not giving him a blowjob or demands you give him a reason is so extremely not worth your affections.
For a lot more accurate and detailed information on cum, pregnancy, sex, STDs, and related issues, go here; it's an excellent site for basic (and beyond-basic) sex ed.