In order to get any kind of decent/effective penetration, you need to keep her legs apart and also her pelvis tilted upwards. Ideally, a pillow under her ass can actually be a real benefit but sometimes it is an odd (though brief) interruption to prop them up correctly and you might not know how to anyway. Don't put her knees over your shoulders necessarily (although that's something to transition into once you get comfortable with her insides), but hook the back of her knees over your elbows and then brace yourself with a wide grip so that it both spreads her wide and also forces her to roll up onto her back a little.
You will need something (spit, lube, whatever) either on the head of your cock or if you actually to somehow manage to get her wet enough that she leaks out a little, that can work. You can't get in without the friction being reduced by some kind of slippricant. Don't be afraid to actually reach down and use your fingers to actually part her lips a little so you can work your way in--don't put your fingers inside her necessarily, but right on each side of her slit and press in hard enough to pull them apart.
Try different angles of attack. For the most part, you'll want your stroke to be angled upwards so that the head of your dick will graze along the roof of her vagina. There's more to it when you actually get acquainted a bit, but that's a decent start.
Also, try to keep your pelvis somewhat pressed into hers. It's not just about your cylindrical dick pumping in and out of her hole like a piston. Chicks have this little thing just above their slit that's sorta folded up inside/underneath some fleshy parts that responds very well to pressure that moves in circular or up/down motions. By grinding into her while still having fun with your dick, you stand a greater chance of accidentally pleasing the chick.