So I downloaded the assets from Unity that have the first person controller and script already set up. I was wondering if there was a quick-ish, easy way to add a pick up ability to the script and make it where the item you pick up can be put in an inventory. Does anyone have any good video tutorials on this? Or what to add to the script? Or should I just start from scratch? All of this can be very basic, I am a beginner. Thank you!
These systems can actually be really complex, and are often built towards a specific style of gameplay. So, unfortunately, if you want to make one, you’re gonna have to do the hard thing and learn! 
If you feel like learning, the components are very simple: tags, layers, lists, etc. However, it’s building a system that can be used over and over and edited easily without collapsing on itself that’s the tricky part. It’s tons of practice, man. Making games isn’t easy, in fact, it’s very very hard.