Shoe stores, Amazon, etc. sell boot leather cleaners and creams.
Wipe boots after wearing, particularly if exposed to water or salt.
Apply 2 coats of leather cream and rub in to the leather.
If they are suede, they sell suede brushes. Go to a shoe repair shop, they will have all the supplies you need.
There are sprays that you can put on them to help protect them. Go to the store where you got them and ask what is best for that particular kind of material.