So it is your first time and you very are excited about it. But remember that being a little anxious is normal. Relax, breathe, go slow, and remember you don't have to prove anything to anyone. You're naked, she's naked, things are going in places that definitely haven't been there before, it's a whole new thing. Also, its not just your genitals but there are several other organs involved in it.
Normally erections doesn't stand more than 3-5 minutes after penetration. What you see in porn isn't the actuality. So do not expect this when you are getting ready for the d-day.
Women, like men doesn't orgasm only through vaginal sex. Foreplay is just as important. Therefore, if you want to satisfy her to the fullest, learn the basics.
Also size doesn't matter. It's time you stop complaining about it and give a better performance. Except in the case of micro-penis, all other measures more than 3 inches when erect.
Don't expect too much from yourself on your first time. You may have dreamt several time about this day, but when it actually happens, it may not turn out well. Do not worry, you get better at sex with time like everything else.
Being cautious is good, but you'll make it risky if you wear two condoms. There are chances of slipping out. Also, condoms can tear apart if 2 are used together.
There is absolutely no safe cycle. So having unprotected sex is always risky. Chances of STDs and unwanted pregnancy are always up in unprotected sex. Making out while menstruation should never be a reason to miss out protection.