I think it’s just fantastic that you are thinking about such good and important questions at ‘only’ 12 years old".keep on doing that!
1. The Bible is quite clear that God wants us to marry Christians (see 1 Corinthians 7:39 - it is addressed to widows but the principle can be applied to all Christians). So if you are considering marrying anyone, you have to consider whether he is a Christian, some one who trusts only in the Lord Jesus for his salvation, and follows Jesus’ word. There are definetly Christians in the Catholic Church but because official Catholic teaching leads people to put their trust in other things other than Jesus you need to make sure they trust in Jesus alone.
2. I understand your longing for your friend to go to heaven, we all do for our loved ones, don’t we? It’s good that you have been trying to engage her about God, and you should keep doing that. However, keep in mind that you need to be respectful and gracious about it (see Col 4:5-6). Besides directly asking her about her relationship with God, you can also make it clear to her your own relationship with God, such as talking about your thankfulness for things He has given you, mentioning your fellowship with other Christians, talking about living your life according to God’s will etc. This might witness to her about the goodness of being God’s child indirectly. But if she is really very resistant, do remember that God is the one who changes hearts, so keep praying for her and trust God in this matter.
3. I’m guessing that your family might not be Christian, which is why you feel uncomfortable? My own family is non-Christian, and I feel that way plenty of times! At times like that, I try to remind myself that Jesus is our great Saviour and Lord who deserves everyone’s worship. So really I should be proud of him instead of being ashamed of him! And I try to remind myself that Christianity’s central message is something that is good, powerful and saves people and so we really should not be ashamed of it (Rom 1:16-17). With family it can be harder because they are so close to us. I find that the more I strive to live a consistent life before family members, and the more I am open about my Christian status, more opportunities crop up to talk about Christ in a natural vein.
4. If you do know and trust that Jesus’ death on the cross brings us forgiveness for sins, then the good news you ARE definitely going to heaven! Jesus’ death totally guarantees our future for all eternity"hear the comforting words of the Bible"For if, when we were God’s enemies, we were reconciled to him through the death of his Son, how much more, having been reconciled, shall we be saved through his life!" (Rom 5:10). Christ’s death has reconciled us to God, and therefore we know now with absolute certainty that on judgement day, we"shall be saved" i.e. go to heaven. And this guarantee is open to any who trusts in Jesus, and does not rely on how we perform, how good we are at all! Isn’t that just an amazing assurance? There are plenty of other Bible passages that tell us that"you can look at John 3:16, John 6: 40, 51, 2 Timothy 1:8-10, 1 Peter 1:3-5