go (do something)

In casual speech, you say "go (do something)" to mean "go (somewhere) and (do something)". For example:

Do you want to go see a movie?

This means to go to a movie theater and watch a movie.

Could you go find me a screwdriver somewhere?

This means to go to the place where tools are kept and find a screwdriver for the speaker to use.

pick up (something)

The phrase "pick up ___" can mean to buy something at a store. When you use this phrase, it sounds like you are buying something quickly while you are on your way to somewhere.

stuff

"Stuff" can mean almost any physical items or material.

It's similar to the word "things", but "things" are countable and "stuff" is not. So "stuff" is better to use when you're talking about a group of things that are different from each other - different sizes, shapes, etc.

"Stuff" is especially used to refer to things that you buy or own:

There's a good chance that you'll end up buying a lot of stuff while you're there.

I'll go pick up all the stuff we need.

Excuse me. Would you mind watching over my stuff for me for just a minute?

But "stuff" can also mean "topics" or "ideas", like in this example:

We have a lot of stuff to talk about.

I'll (do something)

Use "will" to offer to do something, or when you've just decided to do something like in these situations:

OK. Well, I'll take it.

I'll keep an eye out for it.

("I'll" is short for "I will", of course.)

When you've been planning to do something for a while, don't use "will". Say "I'm ___" or "I'm going to ___".

We're going to keep it to just close friends and family.