i m going to have first time sex in my life after 3 days. (age-22) i m afraid whether my penis will get erect or not? will i be able to satisfy her or not ? what are better positions for first time sex ? plz help me

The first piece of advice is: don't drink alcohol. At least not too much. If you drink too much you may have poor erection performance.
Secondly, stop worrying! Everything will be fine. There is nothing to worry about. Just enjoy yourself. The main thing is to stay confident. If you give off confidence, she will admire you for it, no matter what happens.

As for positions, its most likely you will begin in the missionary position (man on top, woman facing up), but for better sex you might want to try the x-position or any woman on top position.

Your main problem is most likely to be coming too quickly. Getting over this comes with experience, and the right technique. Even though you only have 3 days, there is still a lot you can do. Read about the pc muscle relaxation technique and the others in the orgasm control section. Now practice these on your own during masturbation. A lot of improvement can be made in 3 days if you understand how your body works.

Best of luck.