What to mulch my newly planted seeds?

Leaves work, like dried leaves off of any tree or plant but the layers would need to be thick because they will blow away. Newspaper works too, it will keep moisture and heat in but you will need to replace it like every week because it will decompose and rot. Look online and you could find a lot of ideas. Or go to your local garden store and they will have advice.

Make sure you don't mulch till after they've sprouted. A good inexpensive way is to use grass clippings that were not treated or to mulch up leaves with your lawnmower. We pick up bags of leaves the neighbors leave out for the trash company to get and use them as extra mulch. (saves money). If you are really concerned about weeds around them, lay a base of black and white newspaper down and then put the mulch on top of it. Water it well and it won't blow in the wind.

I don't put any mulch on until the seeds come up so that I can see where they are and not suffocate them. Then I put my lawn mowings thickly along each side of the line of seeds. This keeps the moisture in and discourages the slugs when the top layer becomes dry. I also mulch with compost for very hungry plants such as tomatoes and marrows. You can also mulch successfully with shredded paperwork or torn cardboard.