How big is the garden, and what is your budget?
Do you want a 'designer' or a landscape gardener? A designer will come up with a (usually expensive) plan, and will then charge extra for the work. Landscapers will construct all the paths, patio, shape beds etc however you want. They are usually pretty good at coming up with a garden plan as well.
Try telephoning a couple of local garden centres, and ask if they have a list of recommended contractors. The local paper is also worth a go.
Before anyone comes round to have a look (when they can, obviously), make a list of all the things you need, shed, washing line, barbecue, play area, whatever, and the sort of vision you have, whether cottage garden, contemporary, low maint etc. Also make a list of things you don't want! Try and figure out where the best place for stuff like a patio or a bench would be, to get the best of the sun or shade. Work out where the sun is at different times of day too.