This page of The Witcher 3 Wild Hunt has key information about Geralt's equipment and gear. You will learn, e.g. how to manage the inventory, how to deal with the inventory limit, gears' durability, and item slots.
Inventory and equipment parts organization is not something you should ignore. The lack of enough control over those game elements might lead for example to Geralt being overloaded or weakened due to the lack of good enough items. Below you will find various hints and advice about this aspect of the game.
Very important thing that you should know is the maximum capacity. Each item added to the inventory has its weight. While in case of documents or alchemy ingredients those are very small values, a sword, armor or animal skin can weight a lot. Due to that, it's easy to overload Geralt and thus make it impossible for him to run until he removes the item's weight. It's not a serious trouble during exploration as the hero can still move fast on the horse, but when you're in the middle of combat, overload can be a serious encumbrance that makes it impossible to run towards a more far away enemy or to perform any agile movements.
You can "fight" the capacity limit in two ways. First is disposing of all unneeded inventory items after each longer journey. Go back to your favorite merchants and first of all sell them the items that weight the most (1 weight unit or more). Second way is increasing permanently the maximum capacity limit. By default the limit is 60 units but you can increase it up to 160 units. The limit is increased by obtaining Saddlebags - you can find them in the game world, buy from the merchants or obtain by completing some quests. You should take interest in the latter especially - first saddlebags that increase the capacity to 90 units can be obtained by winning one of the horse races in Crow's Perch (side quest "Races: Crow's Perch").
Repairing weapons and armor
Maintaining the inventory and equipment consists not only of keeping Geralt not overloaded but caring for your weapons as well. All weapons and armor parts are gradually wearing off and their current state shown in percentage can be checked by highlighting them in the inventory. You shouldn't ignore this because as items durability is reducing, their quality is lowering as well. It's especially noticeable in case of weapons. Geralt is dealing much lower damage with a worn sword.
Equipment parts can be repaired mostly at blacksmiths and armorers living in the game world. But you must know that each such operation is quite expensive - repairing a very worn item to perfect condition can cost few dozens or even 100 crowns (in case of very expensive items). Because of that, it's a good idea to combine repairing weapons with selling found swords and armors to the person whose services you're using. This will allow you to reduce the costs of service.
Another way of repairing items to perfect condition is using repair kits. Save the repair kits you find in the game for later instead of selling them with other items. Before repairing an item check how much the condition of the item will improve. Don't decide to repair an item too early. For example, if repair kit repairs 40% of the weapon then don't use it if your sword has lost only 20% of its durability.
Quick access item slots
Another very important inventory aspect are six active item slots. Active means you can place there items manually activated or consumables. Moving some items to those slots can make the game MUCH easier and in some situations (for example in some quests) it's mandatory for completing your objectives.
In the two left slots you can place food and potions, items used mostly during battle. You can use them as you wish, for example you can place food to both slots. However, it is best to use one slot for food and another for potions, for example for healing potion or one that temporary increases Geralt's attack power.
To the middle slots you can insert bombs. In this case as well you can pick two similar bombs or bet on diversity.
The right slots are used less often - usually you place there special items, like the magic lamp received from Keira Metz that is used for finding ghosts. You don't need to have these slots filled all the time.