Rock Shox Spring Weight?

At 120 pounds, you'll likely want the "light" spring. I believe they come stock with a "medium," which is usually good for average weight riders in the 150-170 pound range.

If this is your first time replacing your fork, know that you'll need to get a fork that fits in your frame. It should also probably match the travel of the fork you're replacing.

I see that the fork you're interested in comes in several steerer tube options:

1-1/8" STKM steel, 1-1/8" aluminum (-163g), XL 1-1/8" steel, XL 1-1/8" aluminum, 1.5" to 1-1/8" tapered aluminum (-123g), 1.5" aluminum (-146g)

You need to determine if your bike has a 1-1/8" straight headtube or a 1.5 to 1-1/8" tapered headtube. If you can afford the aluminum upgrade it will save you a good chunk of weight.

The XC 28 is offered in 80/100/120mm travel options. Measure the travel on your current fork using a tape measure.

https://www.sram.com/rockshox/products/xc-28-tk-0

When you get the new fork, you'll have to cut the steerer tube and install a new star nut inside the steerer. This can be a little tricky, so consider having a shop show you how the first time.