regarding first intercourse, will it be painful?

That is like asking how long is a piece of string.

First of all some girls find it painful while others don't mainly because some have a higher pain threshold than other, there more aroused before they start, there also more relaxed during penetration and they may have already broken there hymen or may just have a very thin one compared to a thicker one than needs more pressure to penetrate it.

If your not relaxed then the muscles in your vaginal entrance will be tighter constricting the opening from stretching to accommodate the penis there for causing discomfort and maybe pain for you, if your not aroused enough then your vagina won't be ready to accept the penis as it won't of expanded and it won't be lubricated meaning when the penis is inserted it will cause discomfort or pain, the only advice that can be given is to make sure your not going to get disturbed, take plenty of time before you try to have sex just getting to know each other's bodies, use plenty of foreplay before you even try and the most important thing to use a condom as protection!

If you do all that then you should find it will go smoothly and won't be to painful if at all.