Adding to what others have said and (maybe) giving a little more info:
What is it like? For the guy being sucked it's the warm feel of his partner's mouth and seeing how far down his partner can take him. Some guys like a lot of spit and gagging, others not so much. For the guy sucking there's a lot more "mind" involvement: "Holy sh*t! I have the most amazing part of this guy in my mouth and he's letting me suck him." All of the tips above are great regarding pleasuring the entire area including the balls. Also, tell him how much you love his cock in your mouth.
So about the precum/cum: While you're sucking, with the head of his cock in your mouth squeeze from the base of his cock up to the head and run the tip of your tongue over his slit. You'll be rewarded with some silky fluid that can have little or no taste. You should agree beforehand if you want him to cum in your mouth. If you don't, a lot of guys are ok with that. If you do, good choice. If you've done a good job you should be rewarded for your efforts.
What happens? You'll pretty much know when he is about to cum (a lot of guys will tell you). Cup or lightly tug on his balls - they will most likely be pulled up tight to his body. You'll feel his shaft start to pulse as the spurts of his seed go into your mouth. Continue to suck and swallow. As his orgasm subsides gently swirl your tongue around the head. At any point during his orgasm if you moan it will create a subtle vibration, adding to his pleasure.
What does cum taste like? Some like it, others don't. Keep in mind that even if you spit it out, cum is very viscous (thick) so you'll still be able to taste it. Cum can taste different with different guys based on their diet and, believe it or not, whether they smoke. Cum is high in protein and, unless the guy is HIV+ and you aren't on PReP, there's no harm in swallowing it. Some even say they feel a little high/drunk after swallowing.
Good luck!