Any humus rich material will help, including leaf mould. You can buy composted animal manure, or soil conditioning compost (not potting) at the garden centre, and if the clay's that bad near the surface in the top spit (spade's depth) also buy horticultural grit and add that to the ground.
A clay subsoil is not unusual - most of London is on clay substrate, and plants grow in it perfectly well. Just add the humus material to the top spit, and plant anything you like that likes the conditions you're providing, including taking account of aspect (how much sun/light the area gets) and micro climate for your part of the country.

To make use of the autumn leaves: rake them up and either make a leaf pile with chicken wire attached to posts in a square, or put them in old compost bags with a few holes. Water them if they are very dry. Roll down the top of the bag and store it somewhere in the garden. In a year or two the leaves will have rotted down into lovely crumbly leaf mould that that you can use when planting things or as a mulch.
As for what plants to put in your garden, go to the RHS website and search 'plants for clay soils'. They have lots of info on how to improve the soil and a list of various types of plants that you could choose from.

If you can't wait, roses do really well on clay soils!