This is where you need to do some research. Firstly, What do you and your family like to eat? Prepare a list of things that you know you like. If you need inspiration have a browse of any of the veg seed supplier’s websites.
Once you have your list find out how, and what is required, to grow them. You can do this in many ways, if you have any allotment/veg books look up the crops there, search the internet, and/or ask the advice of your allotment neighbours (this is particularly good for finding out what does and doesn’t grow well on your allotment site – For example, carrots can be tricky to grow on rocky or clay soils).
Once you’ve found out how to grow each you should think carefully and decide if you have / will have the facilities / space to grow them. For example, I’ve learnt from bitter experience that it is fairly pointless growing cabbages without netting. This prevents(limits) the butterflies laying their eggs. (If I find the pictures I have of the completely leafless cabbages from 3 years ago I’ll post it here). Caterpillars will decimate your cabbage patch in a matter of days if you’re not careful. So if you don’t think you’ll be able to buy or build protection I wouldn’t bother with cabbages, at least not until you can get some for them.
Now draw out your plot to a rough scale on a piece of paper, mark on your beds, then each of the crops you wish to grow using what you’ve learnt about the space requirements for each. Try to keep crops of a similar type together for now. This will help with crop rotation later. You may want to then cut down or increase your list of crops dependent on the size of your allotment.
Now that you have your definitive list of crops, get online or down the garden centre and get buying. 🙂