I had that situation... and still do.
Theres a girl in my school who i just cant stop thinking about, even though I flirt with other girls ... I know exactly how u feel.
Last year, I did that with someone else, i'd just watch her, always interested in what she was doing, trying to find common ground and how to start a conversation... I did talk to her once, She had a really annoying friend that she was ALWAYS with, and i really didnt like her, she was kinda guarding her, making it harder for me. I finally found a time she wasnt with her friend (I kinda forced it by taking a spot RIGHT next to her so her friend had to go somewhere else) ... We were talking about a school project, and i kept trying to be funny, carefully reviewing every sentence I said over and over again in my head before I said it aloud. Since then, I started talking to her more, since I noticed she wasnt telling me to leave or giving me looks like shes bored. I asked her out earlier this year (broke up a month later but thats beside the point)
My suggestion ... Look for an oppurtunity when shes alone and TAKE IT, if you cant find a time shes alone, try to force it on her tehe ...
Once you make your move to be within her distance, talk to her, and try not to say anything stupid. DO NOT stop talking for more than 10 seconds or she will lose interest in you real quick. Make it seem like you have a lot to share, whether you do or dont. When you have to leave (or you run out of things to say) dont just say goodbye, make sure you say something like "So, I'll talk to you later I guess?" if she says yes, keep talking to her, get to know her, and let her get to know you too.