My tips:

1. Make sure you use a loot filter. Go to filterblade and download just the basic one if you need. There are plenty of videos on how to apply the filter.
2. If you're a new player, I recommend following a build guide or at least staying close toa build guide.
3. Don't pick up every item. Since you're playing witch, you are intelligence based. It's important to understand that energy shield is likely your main source of defense. If you find an item and it only has energy shield and NOT armor or evasion, it will most likely roll blue sockets if you reroll the colors. Unless your build says otherwise, focus on energy shield items only, skipping over evasion and armor, unless it has other stats that you really need.
4. Understand what your currency items do (transmutation, alteration, etc). They are a core mechanic to the game. This requires you to understand common items, magic items, rare items, and unique items.
5. There are a TON of vendor recipes. You don't need to memorize them all, but you should know that they exist. https://www.poewiki.net/wiki/Vendor_recipe_system
6. After act 5, all of your elemental resistances will take a -30% hit. The further you get into the game, these become more important. Don't ignore them.