Pretty much dealing with whatever you find. I don't think it's worth passing up an allotment if one is offered - plus since it's quite late in the season, you'll probably be looking to get a few plants in now, but mainly get it ready for next year. Things to consider:
is it overgrown with grass or weeds, and will you need to lay down a weed suppression membrane or tarp to get it in good shape for next year? If there's bindweed or convovulus it'll be a pain in the arse to sort out.
does it have raised beds etc, and will you need to do any landscaping/woodwork?
what kind of soil is it? Sandy, clay? Is it open or are there lots of trees around? Is it on a hill? All of these will inform what will grow well but a lot of it is trial and error.
Look at the plots around the site - what's growing well, do people have lots of perennial plants or is it all annuals? You might want to get some fruit bushes, rhubarb etc (for example) in soon so they can get established for next year.
Check out the facilities - does the site have communal tools, shed etc, or do you need to buy all that? Ask if there are any rules re pesticides, fires, bringing guests etc. Chat to other plot holder about what grows well and what's not working for them. And bear in mind, everyone who has an allotment loves to give unsolicited advice to newbies, so be prepared for drive-by critique of what you're doing - some of it will even be useful!