I know they are greedy plants and need good nutrients in the soil. RHS says "Grow sweet peas in fertile, well-drained, humus-rich soil and in full sun or very light dappled shade. For best results, incorporate organic matter such as garden compost or well-rotted manure at least four weeks before planting and apply a general fertiliser, such as Vitax Q4 or Growmore, at the manufacturers' recommended dose. After planting, water the plants well during dry spells."
Just bumped up Davids Sweet Pea thread for you Emily...what he doesn't know about sweet peas isn't worth knowing!!!He is our resident expert!
Have they been undercover Emily or are they already outdoors? If the former, get them used to the cooler temps first (hardening off) as it will be a bit of a shock to them. Outside through the day and in at night for a few days and keep them sheltered initially if it's a windy wild site.
Otherwise, you can plant them against a wall or fence where they get sun and/or a bit of shade. Nice deep fertile soil and canes or netting or wires for them to grow up. If you have an obelisk in a border you can use that too.
You can also grow them in containers, just make sure they're a good size and depth. A tripod of 6 foot canes (or similar) for support. Keep them moist and when they start flowering, use tomato food or something similar to keep them happy. I use around 6 to 8 plants in a 15 inch pot - any more than that and they get crowded. They get quite substantial as the season goes on