I have a council allotment of 265 square yards.
I was worried that I would do everything wrong, but I found everyone very helpful that first year. I also joined the same online allotment forum as Jaxjacky, and for the first two years posted questions everyday during the growing season.
You must grow what you like! I also grow what is expensive in the shops, for example my first early new potatoes are chitting upstairs, and they will go in when Monty Don puts his in. From the allotment at the moment I am harvesting purple sprouting broccoli which is delicious and far removed from anything in the supermarket. Both of the above are quite expensive in the supermarket but very inexpensive to grow.
It is hard work, but even so, when you can pick your own veg and fruit all summer, and make jams pickles and chutneys, it's all worth it.
Just go steady and do abit at a time, start with half an hour there, and then build up from that. That way your muscles will get used to it without complaining too much.
What to grow this year? If it were me I would put a few rows of potatoes, some onion sets, and maybe some courgettes. They are all so easy to grow, and won't need netting from the birds, etc.
Good luck, and enjoy those moments when you are digging and the robin is pecking round your fork, and the blackbirds are singing!