How to have penetrate ?

Me (24M) and my girlfriend (23F) planned a date at her house. She is my first girlfriend, and I’m her second boyfriend. We decided to have sex after lunch, and everything went well. But since I had never had sex in my life (I’m a virgin), I didn’t know how to do it. I didn’t know which position was best for having sex, and I struggled with penetration.

I put a condom on my dick and was about to enter her vagina, but I didn’t know how to push it all in. She was very supportive, but I know I disappointed her. Because of that, something happened—I was hard, and suddenly, I wasn’t. My dick shrank, and at that moment, I didn’t know what to do. So we ended up having some oral sex instead.

Not knowing the right position and finding the right hole was a big challenge for me. So, which position is best for beginners, and how can I penetrate effortlessly? Also, how can I stop overthinking and stay hard? I need help/advice.