Congratulations - it's a hugely rewarding hobby I've found.
It's useful to have a shed - maybe you could get one from freecycle?
You are a bit late this year for some things, but carrots & beetroot & lettuce & radish + chard will all be fine from seed as will some types of cabbage/caulies.
If you can get hold of small plants (maybe from local farm shop, or spares from fellow allotmenteers or as meglet suggests ask on freecycle) now is a good time to plant out sweetcorn, beans (french + runner - need canes for both of these), courgettes & squashes, tomatos and leeks and brassicas (purple sprouting brocolli, cabbages, cauliflower, sprouts) + maybe celeriac. You might get away with planting any of these from seeds now (not celeriac), but small plants would be better if you can find them. Rhubarb crowns would be good to plant out now too.
In the autumn plant overwintering peas, broad beans, onions & garlic and look out for people with spare strawberry runners that have rooted. Plant soft fruit bushes in the autumn/winter.
Whether or not you need to add topsoil depends what it's like, but I imagine it's unlikely to be essential. Before planting brassicas add some lime to the soil.