It is really important in Witcher 3 Wild Hunt to have a good gear meaning armor and weapon.
I would start with that you can have witcher swords at the begining in White Orchard the viper swords.
Levels:
Silver sword level 1
Steel sword level 2
When you actully start the game and get to Velen the first witcher gear you can get is the Griffin gear and for that Geralt needs to be Level 11.
The second witcher gear (my favourite one) is the cat witcher gear you can get it near Novigrad in Velen you have to be level 17.
The next Witcher gear you can get at level 20 is the Bear witcher gear in Skellige.

Notice:
For this witcher gear you need a free dlc you can find it in the steam store.
The next gear is the wolf gear at Kaer Morhen at level 19.

Of course all of thesse witcher gears are upragradable.

In Blood and Wine dlc you can upragade your witcher armor to master level and also you can find the Mandragorian armor in Blood and Wine dlc.

In the Hearts of Stone dlc you can get the Viper armor at Max Borsodys auction house and the steel sword in the treasury room and the silver sword if you choose to save Olgierd.If you choose to save Olgierd you get to Gauter'O'dmin game and you can find the sword in there.( Also if you want to save Olgierd you have to speak to the profesor.)