Ok, this will make me sound like a real Yahoo but… When should you use which and when should you use that?

Use “that” when the phrase that follows is restrictive, meaning that it narrows down the group of all somethings into a smaller group. Use “which” when the phrase is descriptive, meaning that it doesn’t narrow down the group.

Both of your examples are restrictive, so “that” is correct.

“These are the people that I was telling you about.” (These aren’t all the people in the world; these are the particular people I mentioned to you yesterday.)

“This is the car that I seek to purchase.” (This particular car is the one I want to buy.)

Here’s a non-restrictive (descriptive) example, using “which.”

“These are my cars, which are very fast.” (All of them are fast.) If only some of my cars were fast, I would say, “These are my cars that are fast,” which would indicate some of my cars (just the fast ones), not all of my cars.

These are the people THAT i was telling you about

This is the car which I sich to purchase

these are the people THAT I was……

and I dont know what “sich” is supposed to be so I cant properly word the next sentence.

“which” is better not to be used for people–for people, you use “who” (or “whom). “Which” is places, or things. (You remember, a noun is a person, place or thing.)

“that” is all-purpose–you can use it for people, places or things. Many people use “that” because it’s simpler.

“These are the people that I was telling you about.”

“This is the car which (or that) I wish to purchase.