If you're talking about using mods, you just find the mod(s) you like at either skyrim.nexus.com or on the steam workshop. With nexus, you have the option of either manual download or using their nexus mod manager to do it for you. With steam, just hit "subscribe" and let it have a few minutes to load, the next time you go to play.
There's a free program called LOOT (took over where BOSS left off) out there, which is helpful in checking your mods for conflicts and determining the proper load order. The short version is that the most recent mod has priority, so if you have a mod that changes Breezehome and then a later mod changes it again, the later mod will "win." You may see problems, or actually crash to desktop.
I'd youtube "loot" and look for a short how-to, so you have an idea what and why, then search both steam and nexus for the mods you like, and decide which is easier/simpler for you to use.
If you're talking about creation your own mods, then you need to download the creation kit, and lean heavily on the various tutorials found on their page or youtube.