1. I've never joined a club but many friends have. I'm told that they're generally pretty keen to have members and will be inclusive of new blood, regardless of age, skill, experience, speed etc...
2. At 19, 29, 39 and older, many of us stop occasionally to think whether we're doing, thinking, saying or wearing the right thing. I think cycling clothes are pretty good for cycling in. If you don't want to look too 'all the gear', you might put cycling shorts on under other shorts and just wear a loose top. I've often done that when commuting with a train journey involved... I'm not really comfortable sitting in an express to Paddington in full lycra - although clearly some folk have no issue with that.
However, I think that if you're doing some miles you do want the right shorts and a pair of cycling gloves.
3. Everyone has their own way of carrying spares and unless you have endless muscles and a fabulous sense of balance you can't take a complete workshop with you. My normal kit is 1 tube, 1 repair kit with levers, 1 pump, 2 Allen Keys. I usually stuff that into a plastic soup carton and carry it in a bottle cage.
Keep riding, keep enjoying it and get faster and fitter. Once you are riding far and fast you really won't mind who wants to take the mickey...