For your second question, ask for feedback. If you do something wrong, chances are he'll stop you or guide you in some way. My guy likes it when I flick my tongue quickly back and forth right underneath his tip, or drag it in circles around the tip while squeezing my lips lightly around him. Idk, it's easier done than explained, haha. But seriously, just ask him if he likes it. Or do like I do and before you start say, "The only sounds I want to hear coming out of your mouth right now are appreciative noises, suggestions, and occasionally my name." ;]] It makes asking him if he wants me to do something more or different less awkward.
More to your second question.
Just don't become a slut trying to get good at it.
If you are about to give a guy head or a handjob, two things;
1) Be quiet as you do it, he won't want to have a conversation with you as you are pleasuring him
2) Hold eye contact with him
You may not feel comfortable doing this, but honestly watch a porno and see what the girl does.
If you aren't comfortable watching one though, do you really think you'll be comfortable with a guy?
In the butt!
I love to be grabbed on the outside of my pants or underwear. I don't know why it turns me on so much. You will learn by experience and paying attention to the guys reactions. There is no set way because everyone is different.
We like our penises to be touched through our pants our stuck your hand down them
Every guy is different, so communication is key.