I understand your struggles with not being able to get a girlfriend. I am in the same boat, unfortunately. :)
Your question can be a bit tricky to answer, as there is a different way to progress to relationships for everyone. However, here is how I would go about it.
First, have a crush on someone! For some, this may take awhile. But when you do have a crush, let's continue to the next step.
Next, build up the courage to ask her out or tell her how you feel. This can be a very hard thing to do, as it is scary and you aren't quite sure how she is going to react.
Then ask her out or tell her how you feel. You hope her reaction is good, and she doesn't reject you. But if she does, just know that it is okay and you should be proud of yourself for trying!
If things go well, go on a date! After you've gone on a few dates and when you feel ready, ask her to be your girlfriend!
I hope this helps! :)