First piece of advice... Spell out your words. Don't use text writing at all when looking for work-it sends a negative message and does not impress potential clients. Pay close attention to punctuation and grammar, as if you were going to turn it into your English teacher. Believe me, a well-written "resume" can work wonders for your image, even if you are 12. Your resume provides your potential client their very first "first impression" of you, so make sure it's a good one!
Two-make yourself a resume-style flier, and post it in places where you know parents visit. You should include your first name, contact info, hours available, and your skills/experience babysitting. Take a babysitter course that includes first aid/CPR, and make sure you include that on your flier as well.
Three- I pay my adult babysitters $10/hr. I'm sorry, but I would never pay that much to a 12 year old... Even if you were CPR certified. At most, I would consider whatever minimum wage in the area is. The main reason people hire younger babysitters is because they are cheaper than older ones. When I was babysitting, I would charge $20 for showing up, then $2 per kid/per hour. This meant that on average, I would take home around $30-40 (the parents usually rounded up to the next $5 increment) It may not see like much, but you will make more money in the long run with repeat business. I wound up having two regulars who, the longer I sat for them the more they payed. I also did reduced rates for regular business... Like one family, I charged $50/week to stay with their two kids for 2 hours after school. That breaks down to $5/hr, BUT it was steady income.
Also... PLEASE be very careful when meeting a potential family for the first time if you are looking for jobs outside a network of people you know. There are a lot of crazies out there, and while you shouldn't live in fear you need to be smart. I'm assuming that because you don't need a ride, your parents are driving you... Make sure they wait in the driveway long enough for you to make sure that the situation is legitimate. (Meet the parents, meet the kids, then wave out the door to let them know.) Also check in at home at certain intervals (I did every hour or so when babysitting for strangers, until I got to know the family.)
If your parents are not driving you and you are riding the bus/a bike or whatever... Make sure to get a trusted adult to drive you AT LEAST to the first meeting and wait for your signal before leaving. Don't go by yourself.