Oral sex is using your mouth to pleasure your partner's genitals. Many men enjoying having their penis licked and sucked by their partner and many women enjoy having their clitoris licked and sucked. As with all sexual activity, if you do not enjoy oral sex, then you should not engage in it as not all people like to do it or receive it.
Ask your partner exactly what they like- you can't be a mind reader. Its helpful to get ongoing directions like "harder", "slower", "higher". Generally speaking the penis can take more pressure than the clitoris, although once a woman is aroused she may want firmer stimulation. If you repeat the same motion for too long the genitals can become insensitive to it or even numb, so do vary the pace, style and site of stimulation.
Many men find it exquistie to have the area on the underside of the penis, just beneath the head (the frenulum) licked and flicked with the tongue. Running wet lips up and down the shaft of the penis is pleasurable - you can use your hands to be stimulating the testicles or penis at the same time.
It's important for the man not to thrust his penis into his partner's mouth as it is most uncomfortable for it to hit the back of the throat. Despite oral sex on a male being called a "blow job", most men do not like their partner blowing into their penis and this should not be attempted.
In most women, the clitoris is highly sensitive and direct stimulation to the tip of it can be painful or uncomfortable - moving your tongue to the sides and backwards and forwards just above the tip is most pleasurable. Some women enjoy the sensation of their partners tongue moving all over their inner thighs, the whole vulval area and inside the vagina; others have little sensation outside of the clitoral area and prefer stimulation to be focused here.
Some people prefer that their partner wash their genitals or whole body just before oral sex, others are not so concerned about this. Some partners are happy to swallow the semen, others prefer to remove their mouth before ejaculation and continue with manual or other stimulation. You decide what is right for you.
Remember that oral stimulation can be accompanied by other types of stimulation too and don't forget the rest of your partner's body and your own too. Most of all remember to relax and enjoy yourself. Your pleasure and arousal adds to your partner's arousal.