how can I be a foster parent for a dog
How wonderful that you want to make a difference in the life of a shelter dog! The first thing to do is decide whether you want to be a foster "parent" for a Humane Society or a breed rescue. Breed rescue organizations often have less money and are completely dependent on foster homes and private donations. Nearly all reputable shelters and rescues WILL pay for any medical expenses for the pets in their care, including spay and neuter.
When you contact the rescue organization about foster care, you will likely be given a volunteer application. You’ll be asked questions about your home, why you want to be a foster parent, etc. After your application is approved, you may be subject to a "home check", where a representative of the organization will inspect your home to ensure it will be a safe place for rescued animals.
Once you’re approved, you can start taking in whatever dogs you like; young, old, special needs, puppy mill dogs… the list goes on. Older dogs tend to be most in need of foster homes, since it’s hard to place them permanently.
If you happen to fall in love with one of your charges, you can even apply to adopt. It’s a wonderful way to "test drive" a prospective pet, especially if you already have a dog in residence.