How do you become a good friend to people when your too shy?

I met people some people that want to be friends with me but i'm scared to open up to them because they might judge me. They are pretty popular and I really don't have as much self esteem as them. But they are always telling people i'm their good friend. When they sit by me or try to talk to me I ge nervous and I don't say anything. I think i'm going to embarr myself. If they say something to me i'll mumble "yeah" or "ok". But i'm just not as loud as they are and outspoken as they are. I'm scared if do something I might do something stupid. So when they try to work with me or talk to me they end up thinking I dont like them and I have a bad attitude. Don't have a ba attitude. I'm just quiet and most of the time I just have a sad or mad look on my face like i'm depressed or have a bad attitude. I just have low self esteem and don't like talking to people or them getting close to me. I don't like looking at people in the face. I might look away when someone is talking to me and they think that I dont care about them or i'm having bad attitude and its not true. I just feel like they might say something mean or judge me so I stay quiet and have people thinking I don't like them. I don't think talking in front a group of people will help me, but how do I become a good friend to someone who wants to be friends with me?