I've covered lots of seat cushions. You need a fabric that will stand up to much use. It should be tightly woven, darker in color, a pattern (not solid) will hide spots, and have some bulk to it. A cotton/polyester blend will hold up well, if it is heavy enough.
Reasonably priced fabric for covers can be found @ liquidation/end of lot shops. Try the back tables in a fabric shop, watch for 40% off sales, as you are going to need alot. Ask at an upholstry shop for discontinued supplies, etc.. Fabric that is used for covering outdoor furniture should stand up well to lots of use. I found a couple of co-ordinating fabrics @ a Walmart for $4.00 a yard.
I used one pattern for the top and sides, and the other for the bottom; making it reversible.
If you are going to use welting, I encourage you to choose a dark color for it, as that will get the most use. Again, tightly woven.
If you are a novice, seek advise from someone who has made covers for cushions. You should be able to check the fabric and feel it to decide if you will use it or not.
Best wishes as you work with your project. Mona Simpson