Most digital kits use .PNG files and JPEG files, so you should choose a program that will support those two file types. Essentially, you can use any software that allows you to work with layered objects, and supports the PNG file format.
You need to work in "layers" when making your scrap pages, so ensure the program you use supports layers.
Here are a couple programs you can use for digital scrapbooking.
Adobe Photoshop-A popular but expensive editing software
Adobe Elements-A cheaper software that does a lot of what Photoshop does. In fact, for digital scrapbooking, this is all you need.
Paint Shop Pro-This program focuses on digital design more than editing photos. Will also allow you to create beautiful pages.
GIMP-A photo editing software that has been compared to Photoshop and the best part? It's FREE. This program supports layers and can open PNG and JPEG files.
This of course does not complete the list of programs you can use, but it will help to get you started. The programs I listed offer a 30 day trial so you can see if the program is right for you before buying.