You need some dog skills, just being able to put one on a lead and go out and come home with the same dog is not enough.
To be financially viable you need to be able to walk several peoples dogs at once.
The alternative is to offer private walks, and possibly do some training - which gets a higher fee but you need far more skills for that and it is less popular due to the higher costs.
You will need a van with secure cages - vans aren't too expensive, but the cages ARE, and the commercial vehicle insurance of course.
You will need your own insurance, dogs can cause very expensive accidents.
You need some understanding of how to run a dog business, for example you can't just load up a bunch of dogs who do not know one another, stick leads on and walk them, you will end up with a dog fight on your hands.
You'll need to understand dog behaviour, have some first aid training, set up a system to assess new client dogs before they go out with you.
The market is pretty saturated, and of course good dog walkers prices will always be undercut by the uninsured, un-skilled twats who will load their car up with ten dogs, drive to a field, let them go, causing fights and drama everywhere... or load up the car and drive around all day and not walk the dogs (or shut them in the conservatory in their own home and go out for the day and the dogs all die from heat stroke... actually happened, she lied and said the dogs ran off in the woods, she had everyone out looking, claimed they'd been stolen, all sorts, when she knew they were all dead.)
These people will not just steal clients from you, they'll stop people ever being your client because even though most people cotton on and stop using them... they're usually so put off dog walkers they won't ever use another!
It does on the surface seem an easy way to earn money but it isn't, and it's also pretty unreliable income wise - this summer all my dog walker friends lost thousands because it was too hot to walk dogs, and no walky... no money. Even with going to clients homes to let dogs out for wees, they didn't make the money they were used to making during those weeks and many people didn't need them to do that at all.
There are also days the weather is just too horribly wet and windy to take dogs out and again, you'll lose money there.
You really do need to love dogs and actively want to work with them, AND have a good understanding of the professional dog industry and running a small business. I am not saying don't do it... im saying do your research!