Don't start breeding rats and mice. Once you've done a lot of research, you have a good vet, some extra space, and some free income, then breed rats OR mice. If you get that down maybe you'll be able to branch out into the other but it's a huge responsibility so don't get ahead of yourself.
Also be very careful about where you get your rodents from. Many of the rodents in the pet trade have very undesirable traits and diseases that can be passed on. Never breed unless you can say for sure that there are strong healthy rats going back 3 generations in a line.
My next tip would be get everything like your supplies, some extra cash, your vet, and a few potential customers lined up before you breed. You don't want to be stuck with a pregnant mommy in a vet emergency and no good vet. Or stuck with a whole litter and no one who wants them. You have to be prepared to keep all unwanted babies yourself to ensure that they don't go to petstores and end up as snake food!
Finally do all your research about which markings can be a warning sign for megacolon, normal gestation periods, signs of pregnancy or labor gone wrong, care of babies (you may end up handfeeding every two hours. I have.), care of pregnant mothers, etc before you start. Good luck. Let me know if I can help!