My advice? Go with your heart.
Fostering is work. You have an animal to take care of and that includes feeding, bathing, cleaning up after, and taking to adoption days or the vet. (Some rescues will do the vet visits for you.) Most rescue volunteers will pitch in and help each other with adoption days and if you have a dog that requires professional grooming, the rescue should pay for it.
Choose the right organization to work with for fostering.This can be your local shelter. Most accept fosters due to the over crowding. Check out their website, visit, or make a phone call and find out what they're looking for in a foster family.
Check out your local rescue. Look them up online. They should be a verifiable non-profit. Go to your state's website and there should be a place to look them up to see their non-profit status. In New Jersey that would be at the Directory of Registered Charities Making sure the rescue you choose is a non-profit helps to ensure all funds and donations will go to the care of the animals.
Once you find your rescue ask about their policies for fostering. Who pays for what? A good rescue will supply all the supplies (food, leashes, medical needs, flea prevention, etc.) and host adoption days in your community. Ask about what happens if you can no longer foster? This sometimes comes up if the dog you're foster doesn't get along with another pet in your home or if you have a family emergency and can no longer keep the dog. There should be policy in place for emergencies.
A good rescue usually gives a first time foster an easy dog. By that I mean one that is calm or maybe a puppy. Most puppies have no issues and just want to play. Good choice if you have kids. Some in the rescue I belong to only foster puppies and others only want to do the older dogs. I've done both. Puppies can be fun and older dogs are usually already house trained. There are good points to any age dog. Puppies are usually adopted faster than older dogs.
If you can't foster there are always other volunteer options.
Most good rescues run fundraisers and the more hands to help the better.
Processing paperwork for adoptions.
Doing home visits.
Check out your local shelter or rescues. Their website should have a list of volunteer opportunities.