Personally I'd just say start small and learn yourself.
This also heavily depends on your actual dog as well (partially) on his personality. Whether he's off-leash, where you go, etc.
Boots are often one way to protect the paws from harmful influences such as snow, ice, or salt water. Ever seen wolves with boots outside fairytales?
Try to learn how your dog reacts to different ground, then adjust accordingly.
If your dog gets problems with the paws directly related to the ground, get some boots. Otherwise I wouldn't bother. Your dog should feel far more comfortable and have a more secure stand without.
Other things like swimming vests I'd make it dependant on where you go. If it's a small dog or you expect fast flowing water, I'd think about getting one.
Only thing I'd really take with me is some good first aid kit for dogs. These usually include everything you need in case of cuts or other small wounds. Also plan ahead regarding potential vets near your hiking location, save their telephone numbers and addresses.
One more thing to consider might be local regulations. It's possible you're not allowed to have your dog off-leash during some time off the year and you might have to have clearly visible straps on your dog so hunters can associate it being with someone rather than straying.