To get the most from your crops you will need to provide a constant supply of nutrients. Adding compost and even well-rotted manure to the soil will only add small amounts nutrients; they are mainly used to improve soil structure.
Feeding with a fertiliser will usually be necessary. For most crops, add a general fertiliser a week or so before seed sowing or planting out. You can use Growmore, or blood, fish and bone if you garden organically.
Some crops will need additional light feeds throughout the growing season, and this is usually best given as a liquid feed. For leafy crops this should be a balanced feed containing equal amounts of all the major nutrients. For fruiting crops, you should change to one that is higher in potash - such as a tomato fertiliser - to improve the quantity and quality of the fruit.
Plants growing in containers will be totally dependent on you to supply nutrients. Most fertilisers contain enough nutrients to sustain five or six weeks of growth, but after that time will need feeding. You can either feed weekly with a liquid feed or add a controlled-release fertiliser to the compost at planting time.