Babysitting jobs come from word-of-mouth plus personal relationships. You don't get jobs by posting your name and number all over the telephone poles in a city. People want someone they know, or know of, to take care of their children. Therefore, the way to get babysitting jobs starts almost always with people you know.
A caveat to this would be joining a company that hires out nannies and babysitters, such as Sittercity or Care.com. A smart move would be to apply to work for both organizations, as well as continue building your babysitting repertoire through sitting for folks who know you. Just remember that knowing the family you're sitting for goes both ways—they know and are comfortable with you, but you know them as well. Always keep your own safety in mind.
Once you've had jobs babysitting your siblings, other family and neighbors, start spreading the word even more. Ask your neighbors to tell their friends that you're interested in babysitting. Think church and school: if you go to church, you can post a flier with your name and number outside the church nursery or the Sunday School classrooms for children.
Think of your elementary school: the teachers you had know you, and the ones who didn't teach you at least know who you are. Go to the teachers and ask if they'll let their students' parents know that you're available to babysit. Most teachers send out some sort of newsletter periodically, email blast or hard copy. Ask about that and whether they may put a note in there telling parents about your availability. If it's been a while since you went to elementary school or you live nowhere near it, think of friends who teach school and ask them the same thing.
The best way to get babysitting jobs is to accept them. This may sound obvious, but there is a funny trend in calling new babysitters. Parents and caretakers tend to only ask once or twice for the first time. If you are unavailable those first times to sit, they will likely consider you "too busy" to keep on the list of sitters to try. Of course you cannot always control if you've already made plans, but if you really want to expand number of families for whom you babysit, try to make it work when they ask you that first time. One idea, if you're unable to sit the first time they ask, is to schedule a time to come meet the family. That way they'll know you're truly interested in helping out.