Where do I go from here with this girl?

Ok so there's this girl I like, her name's mandy, and I know that she likes me too. My buddy's girlfriend is her best friend and she told me that she liked me. 2 weeks ago we all went out to the bars and had a great time, dancing, laughing, etc..But then I kind of ****** up. I was at her house for pre-gaming and I went over to her and started talking and bullshitting whatever and had a decent conversation. I got up and left for a few mins and came back and there was this dude sitting with her and had his arm around her.At the time I didn't know but apparently this dude's a clinger and she has no interest in him whatsoever. Anyway I proceeded to text her and talk to her that way while that dude was over with her. Then I asked her to lunch the next day and she said yeah she'd be down. Yeah i know I should have just went over to her and kept talking and i was stupid and I ****** up. But I guess she decided to give me a second chance because a week later she txtd me while I was working at our college's library and she asked me if I was working because she was there studying. So we txtd each other for awhile, bullshitting about finals coming up and **** like that. She told me about how she's taking a sports psychology cl over the summer and I told her I could have used one because I played tennis for the school. Then she said she would love it if I taught her how to play over the summer and that she would tell me about what she learns in her cl. So i told her it's a deal and then she mentioned that I still owed her lunch. So after my shift I asked her where she was so I could come talk to her for a little bit. So I went and met her and we just had a good conversation. Remembering how I ****** up by asking her out over a goddamn txt I asked her then if she wanted to come out with me and my buddy, his name is jeremy and he's the one who's girlfriend is her best friend, to the bars. She said she would so I was pretty stoked about that. Anyways the next day I went to a local pizza joint with Jeremy and his gf and I didn't know but Mandy came too. We had a good time, we were joking with each other and blah blah blah, I found out that she couldn't come out with us later though bc she had work to do, so whatever it's cool. Now my question is how do I go on from here. Do I txt her and ask her to go out to lunch, because she mentioned I did still owe her, but then would I just be ******* up again? Or should I just wait until Jeremy and his girlfriend go out to the bars because when they go out mandy is almost always with them and talk to her then? I would really appreciate any feedback, I really want to make this into a relationship, thanks guys