Start sowing vegetables this week as long as the weather is good: it's the ideal time to get most hardy crops into the ground. If it's cold or the weather is wet, though, hold off another week: later sowings will catch up quickly, and it's better not to risk losing your seeds to damp or frost.
You'll find seed for lots of crops you can sow direct now in your favourite garden centre, including carrots, lettuce, beetroot and turnips. Look out for early varieties of pea, like 'Meteor', and broad beans: both are easy to sow in double rows around 10cm across: space the seeds 5-8cm apart. It helps to put your supports in as soon as you've sown them, as then the plants have something to scramble up straight away: twiggy peasticks, or just canes and netting do nicely.