Note: Ice and Fire for 1.15.2 requires citadel instead of llibrary. There is a link to it on the CurseForge page, so you just need to make sure you're using the right dependency.
Many people ask this question, so I decided to write this to provide some assistance. This should work for any mod, not just Ice and Fire.
Note Pocket (Mobile) and Bedrock (pretty much everything but Mobile and PC/Mac) Editions are not moddable.
First, you'll have to buy Minecraft. If you've never played it before, try it without any mods to get used to the game. Ice and Fire makes it a lot harder.
After you have Minecraft, get Minecraft Forge. You need this for most mods, because it allows them to work without modifying the game's code. The official website is scammy and will try to put viruses on your computer. If you don't know how to get forge look up a tutorial.
Then you need to get the mod. It uses Llibrary, so you should download that first. The Ice and Fire CurseForge page is right here. It has a link to Llibrary.
Note that you should only get mods from CurseForge or another website you completely trust. Other websites illegally repost mods to trick people into installing viruses.
Once you have Ice and Fire and Llibrary, put them in your mods folder, under .minecraft. Make sure you have the Minecraft forge version. Then you can hit play and enjoy the mod.