I feel like i keep screwing up?

i feel like i am making a complete fool of myself. ok, when my gf and her friend get together, they have a ton of random conversations that i know nothing about. they joke around about things i dont get, so i just sit there listening. but it always the 3 of us, so im always just sitting and listening. im afraid that this will make me seem boring and uninteresting, so i try hard to get in those conversations, but i always end up saying something stupid and making a fool of myself. and when me and my gf are alone together, we dont talk nearly as much as she does with her friend. im more of the quiet type and they are more of the talkative and random type. i dont know what to do. it doesnt feel like we're as close as she is with her friend as well. i feel like its because of what ive just mentioned. i guess i feel like the third wheel sort of. any help? like today, there were two chairs and a couch at the student center. the chairs are beside each other and the couch is several feet away. her friend sits in a chair and i sit on the couch. my gf sits in the other chair and i find myself just sitting by myself and listening. but when i go to leave, she always walks me out and kisses me. any advice?