You can start by making a list of all the things you think you can't do because you're fat. Or things you haven't tried because you think you are too fat. Make sure this is a list of things you actually want to do and not things you just use your weight as an excuse not to do because you don't actually want to do them. We do that... use our weight for an excuse. It needs to stop.
With each item on the list think long and hard about how your weight is a factor and if it is really your weight or if it is your pride holding you back. Make a new list for each item on how you could possibly alter the activity so you can participate. Anything is possible.
For years we thought (or it was suggested to us) Yoga was only for the meek and sinewy men and women of the world. We now know otherwise. Yoga is for all shapes and sizes including ours. It involved someone stepping outside their comfort zone and creating a new way for themselves (and others) to participate and benefit from participating. It can be done. If you can't think of anything try brainstorming with other fat people. Brainstorming is a great way for producing ideas, some bad but some truly amazing.
Stop using your weight as an excuse for not living life. Get out there, be seen, be heard, live life and enjoy it. Connect with other fat people and spread the contagion of trying new things to them. Spread the word and the enthusiasm. We can do anything we set our minds to and that is really what it is about, our minds, not our bodies. Don't let your mind hold you back from the things you really want to do.