Saxy's answer wasn't to dishearten you Tom - but it's true that as well as being helpful, advertising yourself on the internet can be dangerous. What if someone'd answered you, and asked for your address, etc? Try locally first of all, or approach people that you know with animals. Word of mouth will often get you more work than you expect. Best of luck.
Tom - keep safe, don't put your age or location online as this attracts paedo's. Sadly this is the times we live in.
Talk to local people you know, offer to do stuff like garden tidying or errands as well as dog walking. But let your folks now who you are in contact with.