Lots of fruit, vegetables and nuts. A good variety of these and you should see a big health improvement. Oily fish a couple of times a week is also good because of the omega oils they contain. Go for fish like sardines/pilchards or mackeral. Tuna has some oils but not the tinned variety, this has almost zero. Tinned pilchards are great, including the ones in tomato sauce, great on toast. When you cook your vegetables try to cook them for as little time as possible or you will lose all the goodness. There is a lot you can do for a healthy diet and this is just the start, hopefully others will add to this.
Lastly, avoid trans fats, and cut down on saturated fats.