Coolant Type Brand???
 

Went to Autozone today and they gave me Prestone 50/50 Ready-to-Use Prediluted Extended Life Antifreeze/Coolant. It says its ok for alumninum engines. Is this what I need to be using to top off my Coolant? It currently has something pinkish in color in it. It has been in engine for about 8 months.

No, you need to flush what ever is in there, and replace it all. And forget the extended life. Swap every 2 years or 30,000.

Is there a sticky on how to perform a thorough flush?

I got a new/used engine (22k on it at the time) put in in January so it got new coolant at that time. It's just low but the coolant only has about 8k on it. If I want to top off as it is a little low should I just use water then?

If you don't want to flush it, then yes, it sounds like it has OAT which is the pink.
The best thing to do is call the person who did the engine work and ask what they used.
Here's some info to get you started; Antifreeze Facts

Only use distilled water when needing to add or flush your Disco.
The biggest killer of our radiators is calcium from tap water.

The calcium from tap water, and even worse from private well water. And bottled water will usually have mineral content to make it healthy, just not for your radiator. Distilled is the way to go, and mixing coolants of different brands is not always the best idea. Our pals at Iffy Lube will top off your coolant with something. But maybe not what is best. And radiators that have not been flushed will build up calcium, even if water pump or engine is replaced, still have the old radiator in the loop. The calcium Mike speaks about is those flat chunks in the rows, it can also grow on the walls of the radiator.

There's no harm in using tap water to flush your cooling system, if you really want to flush it. You can use distilled water and change it a bunch of times, but that doesn't really flush it, it merely runs a few water changes through.
For a home flush, when you don't want to spend $500 on something like the Gates Power Clean: Gates - 91002 - Power Clean Flush Tool
you can use one of these that do an acceptable job: PEAK Flush & Fill Kit (other companies like Prestone have the same thing.
Drain everything again and then run a cycle or two of distilled water if you want.

If youre changing over to green the common choice is PEAK 50/50. Harbor Freight seems to have the best price, I got mine there for 6.99/gal.

I buy the un-diluted then mix it with 50% distilled water. It is only a few dollars more plus $1 for the water and you get 2 gallons.