do an archive search on soup and there's lots of lovely recipes already here on Mumsnet.
Also, you can but "broth mix" or soemthing like that in Tesco (or I got mine at Julian Graves) which is lentils and peas etc and you just chick a handful in to any soup to make it a bit more wholesome - very good for breastfeeding Mums!
3 Large Leeks
3 Large Potatoes
1 Vegetable Stock Cube
Milk and/or Cream
Chop up leeks (using as much green as possible) and saute gently in a small lump of butter. Cook until soft.
Wash, Peel and slice two potatoes and add to cooked leeks with 1 - 1 1/2 pints of vegetable stock. Simmer until the potatoes are cooked. When cooked, take off the heat and majimix or sieve to make a puree. Return all to saucepan and add seasoning and add 1 - 2 pints of combined milk and cream.