You have up to 20 Inventory spaces to carry something, so you'll want to make sure you keep a record so you don't lose something valuable, like a key to somewhere or something. Naturally, certain items, like Luggage bags and such, will help you carry extra space, and stuff like pens and unimportant books won't take up space. There is no need to record them if you don't want to...
Here is a following list of items types and what space they take up:
Healing Items/Other Objects - 1 space
Light Melee Weapon - 1 space
Medium Melee Weapon - 2 spaces
Heavy Melee Weapon - 5 spaces
Light Firearm Weapon - 1 space
Medium Firearm Weapon - 3 spaces
Heavy Firearm Weapon - 5 spaces
Extra Space Items:
Gun Holster: Eliminates gun from Inventory if Light or some Medium.
Ghost Hunter Case: Can either carry all Ghost Hunter Equipment on hand for the equivalent of 1 Inventory space or hold up to 5 Items. (Ghost Hunter Equipment will have the Words [Ghost Hunter] on them.)
Toolkit: Toolkits can either hold all tool items, or add up to 2 Inventory spaces to your inventory (Tools will have the word [Tool] next to them when found)