Well a brief explaination for a bitmap would be, oh... say...
You drew a picture of a tree, rock, whatever on MSpaint and you decided to save it. You could save it as a JPEG, but since you want a bitmap, SOOOoooo... save it as a bitmap.
Edit* Ah, I see I'm abit too late. My description wasn't very good either.
i guess thats a legitimate question for someone 12 years old. if youve ever drawn a picture in mspaint and saved it youll notice the file extension .bmp
.bmp stands for a bitmap file. and i would hope that any paint program you use can save a picture as a bitmap. so just use whatever program you already use, like mspaint.
Watch out if you have Vista though. Vista's MSPaint automatically saves in the JPG format, which uses lossy compression. Go with BMP (lossless but big) or PNG (lossless and small) if you are doing pixel art.