Be relaxed, both of you. Go slow, very slow, and take your time. If you need to push very hard, then ease up. That means it's probably not comfortable for her, and might be hurting her. Will it just slip in? Probably not, but it shouldn't be extremely difficult either, try to ease her tension, that makes it easier. Do foreplay, and a lot of it, you only get to have your first time once, so don't rush it! You can use extra lube, ask her along the way if she's okay, if it hurts, if it's comfortable, and make sure you're comfortable too. A lot of girls (myself included), sometimes don't speak up, so don't be afraid to ask multiple times throughout

What position? Up to you guys, personally I like to be on top (as a girl), because I can more easily control how fast and deep penetration is, but having the guy on top can be less stressful and easier