You got a good head on your shoulders, and congratulations for joining the Church.
That’s great that your girlfriend is looking to you for spiritual advice. The earlier in your relationship that you can make a habit of doing things together like praying, going to church and exploring the faith, the better and stronger your relationship may be. That’s a wonderful foundation!
Check out a book called, “The Good News About Sex and Marriage” by Christopher West. Theology of the Body for Teens is also another good book; I know it says ‘teen’ but it breaks things down compared to the heavy academic writings.
It’s not necessarily explicitly just sex that has to wait until marriage but also things like petting and getting hot and heavy (which are mortal sins). Simple kisses, hugs, holding hands are ok. A good way to think of it is “how far would we go if our parents or grandparents were watching.” Unfortunately many folks think (as I used to) that everything BUT sex is ok as no one really explained things to us.
Going to frequent confession is helpful (you get many graces as well as learn about yourself, habits, etc.), as is having an accountability partner such as a friend or someone at church who can encourage you. Take advantage of the sacraments.
Living together, spending the night, sleeping in the same bed, taking trips where you would be in the same room while sleeping, etc. isn’t a wise direction to go as it could lead to sex or petting and sets a bad example for others. Same thing if you place yourself in a tempting situation- it’s easier if things aren’t set up to allow things to get out of hand quickly.
Sex and petting are ok, just within the context of marriage. Just remember, it may be challenging but it’s worth it in the end!
Regarding proposing- communication and prayer are the keys, you’ll both know when it’s the right time. Your priest will let you know about marriage preparation requirements, such as a weekend retreat, evening classes or a mentor couple. It’s good if you both have a rapport with him (start saying hi after Mass, send an occasional email, ask if he wants to go to eat sometime with the two of you), so you can have a good friendship/resource and get the most out of the marriage prep process.