Your question is kind of vague.
You need to own Minecraft to play modded minecraft. You'll need to create a profile that includes your minecraft login info so the launcher can download and manipulate minecraft on your behalf.
Beyond that, modded minecraft offers a metric boatload of content, functionality, changes, etc. I'm a hardcore player and software engineer and I often need to watch videos or series on mods to learn how to use them. So how to figure out which mods you want to research?
Revelations isn't much different from vanilla in flow - do whatever you want and explore mods. If you're not familiar with the JEI (just enough items) mod it's a good place to start since it's a mod that helps you use all other mods (it's an inventory mod you can use to drill thru recipes, find uses for items, find out how to craft or find in the world the various items, and you can limit what you see for crafting to a specific mod and more. JEI isn't terribly complex itself but learning how to use its basic features makes enjoying all the other mods in a pak far smoother. It's also a way to figure out which mods to explore (see which mod an item belongs to, if interested in figuring out how to use it, pursue).
You might want to start with one of the "training" paks instead of Revelations (I love Revelations but it's loaded up and may be overwhelming if you don't know where to start) - there are a few that have a built in quest system that will introduce you to mods in a progressive manner - you get introduced to concepts that can build on each other and/or get directed to do things within your current "tech level"
Another option is to watch some let's plays. Direwolf 20 has a pak on the launcher and always does an accompanying series that pretty much goes thru the various mods and illustrates how to progress. He's a good presenter and good at explaining. He also does a lot of mod spotlights (as do many others) for learning all about specific mods and how to use them.
Youtube is your friend for modded minecraft but there are megatons of minecraft videos, so poke around until you find a presenter you like.
Enjoy!
