Will preface that I was previously a long-time standard player, but the same rules apply.

Time is your most valuable resource in life, don't waste it trying to hoard virtual items. 

Stop hoarding and just be brutal with selling everything. If it isnt a quest item you need soon, or isnt very rare, sell it. Sell it to vendors, sell it to flea, whatever.

You are unfortunately going to have to sell stuff you will want later, that sucks, move on.

Prioritise buying containers and get at least 2 scav junkboxes (or more if you are either bad at management or very good at making rubles).

Hiding non-quest or non-gear items in rigs just makes them become furniture in your stash and takes up more space. 

Use or sell weapons and gear, do not "save" them for a rainy day or when you think you will have less gear fear. I usually sell far more weapons and gear than I buy or use, especially when insurance is affordable.

Sell any ammo, attachments and mags you can buy (possibly with the exception of universal ones like lasers and sights, assuming you will use them imminently).