A few things to say here...
On the weight: If you feel that looks are the reason you aren't being taken seriously by girls, then change what you have control over. A body is something that men have a lot of control over, as a good body pretty much ends at being fit. It sounds like getting in shape would provide you with a lot more confidence, and that is something girls really appreciate. Don't take that to mean that confidence can be a replacement for looks, because while that is a wonderful sentiment, it does not hold a lot of truth. It does mean that you can be in shape but if you are still very quiet and passive, we will still see you as the nice guy but not a potential date/boyfriend/whatever you are aiming to be. Taking the initiative to lose weight will be seen really positively by the girls that already know you - it takes a lot of drive and motivation to lose significant amounts of weight, and that shows a lot of strong character.
On not feeling very attractive (face-wise): You can't change your face. This is something that is very difficult to accept. However, a great personality, some confidence, and some humor are all great things that you can have. You define yourself as a funny guy - that's really big. Don't spend so much time thinking about what you don't have and think about what you do have and what you can have.
On never having a girlfriend: I don't know what this will mean to you, but I'm a senior in high school too and I've never really been in a relationship either. I always end up in flings or "open" relationships or things like that, but never had a real boyfriend. I know girls (and guys) of various levels of attraction, some very unattractive and some beautiful, and I have to say that happy relationships are not based off of looks. Meaning, I know some really unattractive people in relationships who are very happy, and some really beautiful people who have never dated before. It's mostly about finding someone who you mesh with. Don't get too hung up on the one girl - there are so many people out there, and I often feel a little lonely or like I'm never going to meet any new guys so what is the point.. but I always end up either meeting someone new or looking at someone I know in a new light. I'm a relatively happy, flirty person, and I don't regret not dating anyone so far because I haven't found anyone that I really wanted to be with. All these flings and hookups and open relationships suffice for me, I'm not in any rush.
TLDR: You're young, don't worry, be happy.