Don't be too ambitious for this year as it's not a good one. Spend a bit of time reading and work out how you're going to lay your plot out, what sort of paths you're going to put in, where your compost heap will be.
I'm assuming the allotment needs clearing, start doing it a little at a time. Once you've cleared a bit, sow something -lettuce, beetroot, pak choi. Also think now for winter, see if you can get some leek seedlings, sow some sprouts, cabbage, rainbow chard and kale, or you might be able to buy small plants.
In the autumn think about putting in soft fruit bushes. Gooseberries, raspberries, rhubarb etc. my friend has planted hers through weed membrane which I regret not doing as it really is keeping the weeds away. Autumn is also the time to plant garlic and onions (check they are the over wintering kind) plus some varieties of broad bean and peas.
Most importantly, make sure you find a place where you can sit and enjoy your hard work !