Hi Tina, welcome to The Planted Tank!
You will read many opinions on substrate, there are many options between cheap and expensive.
Probably the simplest would be to buy a few bags of Flourite or Eco-Complete, depending on your color preference.
If that is too expensive, there are some cheaper things available with similar characteristics, like Soilmaster, or Schultz/Profile.
On the bottom of the price scale, there is sandblasting grit/coarse sand/fine gravel.
Lots of choices. Depth of substrate depends a little on what plants you want to plant, some are root-feeders and need fatter substrates, like Sword Plants. Others, stem plants, ferns, mosses, won't care since they take their nutrients from the water column. On average, I'd say 2in in front and 3in in the back are sufficient.
Regarding the color of your background, that's your choice... Some think black looks best, I use mirrors in some tanks, it comes down to personal preference. Look at a few tank photos and see what you like...
PS: I moved this into the Substrate forum... here is where you find all the nitty-gritty of bottom layers.