It's easy to run out of space to carry all the essentials (and junk) that you pick up as your blast around in Starfield, so it's a good idea to put some of it away in your ship's hold until such time as you wish to equip or sell it. Let's take a look at how to do this.
Starfield: How To Store Items On Your Ship
Using your very first ship as an example, you'll find the access point for your cargo hold on the left side of the cockpit, as shown below. Once you interact with it you'll bring up a simple menu system which allows you to switch between the cargo hold and your own inventory and you can then get busy switching between both. Simple!
Remember when doing this that each ship has limitations in how much it can hold and it's not a huge amount so you'll still need to sell stuff off at Trade Authority vendors and shops to keep on top of it all.