Almost all annuals can be started this week. Seed packets give good instructions, but here are a few other points to observe. Make sure that you practice squeaky clean hygiene when sowing seeds. Clean containers, clean water and clean compost are essential. Some gardeners like to use spent compost, but unless it is sterilised you're taking a gamble. Larger seeds can be sown in pots - individually, in pairs or in trios - while smaller seeds can go into trays, modules or shallow pots (cottage cheese tubs with holes poked in the bottom are ideal).
Never sow the whole packet: you'll have back-up seeds if you have a disaster, and, in any case, you probably won't need the dozens or hundreds of seedlings that may arise. Sow thinly to allow the air to circulate between seedlings. Water very gently with a fine-rosed watering can, or with a plant mister (although this can be tedious). When the seeds sprout, don't be afraid to throw out excess numbers as you prick them out.