Maintenance is not only something which is necessary to perform on your equipment, it is also a skill which will increase in level as you do so. Initially you will have a limited capacity to repair your own armour. As a result you will have to pay specialist vendors such as Blacksmiths, Weaponsmiths, Tailors, Armourers and Cobblers to do it for you. From each of these you can also purchase the corresponding repair kits, ranging from small kits which can be used on inexpensive equipment to regular kits which can be used on almost all equipment once you have reached Maintenance skill level 20. The more you repair your own kit the more you will level the skill, but you will also gain additional benefits related to specific perks, as shown below.
Note: Keep in mind that repair kits are extremely fragile. At the beginning you may expire one kit in perfect condition after just one repair. As you level you will be able to use one kit for multiple items, but they are still vulnerable to sustaining damage quickly. It is therefore a good idea to carry 2-5 repair kits of each type, depending on how often you get into skirmishes.