Need a program to make flats

I hate asking you guys for things but do you know any programs that makes making flats easily?

Depends on what you mean?

If you want a program to edit images, The GIMP is the best free software. If you want something to add resources, you should use SLADE. If you're looking for artistic or technical help, or something else, you'll need to be more specific.

Ok so here is what I want,I want to put my custom textures and use them for floors and ceilings and if you can do it in slade that would be COOL too.

If your textures are made already, then SLADE is definitely what you want.

Here's a pretty good tutorial that should get you started.
http://slade-editor.wikia.com/wiki/How_to_import_textures#Turning_images_into_textures