Hi
@xji3rctoroi,
It's great you want to have your simselves meet each other! :D
First step to make two Sims interact in the game would be throwing a party. When you throw a party, all your friends get an invitation to it.
If you throw a party, your girlfriend would need to accept the invitation and come to your party. Once her Sim is in your house, make the Sims meet and add each other to their favorite Sim list.
From that moment, you will be able to invite her Sim to your house or viceversa by pressing the 'Socialize' button in the relationship panel of your Sims. Start a romantic story with her Sim and... increase that relationship until you can marry! 🙂
Happy marriage!
-Mai