using a propagator

I am relatively new to gardening in my own first garden though I have grown herbs, tomatoes and some flowers in rented accommodation before. I now own my own garden and I want to grow some of my own veg, though it is not a huge garden.I do not have a greenhouse but have been given an electric propogator. When is the best time to sow seeds in it, so that they will be ready to plant out in the garden when it is warm enough to do so.?