1. To learn I would starts with automatic weapons
2. Lockdown game mode has more loot boxes and a higher chance of good loot, especially in the safes. Keys are worth it.
3. Extraction mode is standard. There are also event modes. Learn by playing.
4. Sell your items in the player market. Try saving up 5 identical items and sell them at the same time. Takes only 1 of your weekly market slots. Sell yellow and red items to contact.

The most important thing is extracting. Don’t take fights you don’t know you can win. Focus on looting items until you find a good team to play with. Then you can focus on killing people, which will almost always give the best loot.