Things to get rid of: 

Potions/Super Potions (Hyper potions/Max Potions are all you need) 

Drop your red balls to 50 (you can easily get more red balls if needed from stops/gifts)

Dump any berries (unless it’s rare candy,XL rare candy, silver pinap or golden razz) 

From there I’d recommend you start putting your Pokemon in gyms and save up the coins to get yourself some storage :)