There's a rather large selection of mods that you can find on planetminecraft.com and often times the instructions for installing those mods are in the forum if the mod does not automatically come with an installation setup to make it even easier. You can also look up feed the beast, which is essentially an installer and launcher for several mod packs (several mods that work well together and change the nature of the game to better suit your preferences) though a lot of the feed the beast mods do require you to be in an older version of minecraft such as 1.6.4. Changing your minecraft version is easy though, when you first load minecraft and start on the homepage with the update news, in the bottom left corner there's the option to update your profile, when you click that there's an option about halfway down about what version of minecraft you want to use, default setting is to use the latest version which as of now is 1.8 but when you click on it there's a dropdown box displaying the various versions you can choose from, the mod will tell you which it runs on. A lot of mods also require you use minecraft forge, but will often times install it for you as well, though it counts like another profile this can also be chosen in the bottom left corner of the initial startup page. If you don't want to deal with installing actual mods there are multiplayer servers that use their own plugins to alter the game and generally you don't have to install anything to run these servers. I will warn you however that a lot of these servers got there plugins from a site called bukkit, and bukkit is currently down so a lot of servers are now experiencing some glitches with the plugins they used to use, overall the plugins do still work but not as smoothly as they used to and it may be a while before they run smoothly again with the recent changes to minecraft such as bukkit going down because former developers are reclaiming their mods and mojang (company which owns minecraft) being purchased by microsoft, and notch (creator of minecraft) choosing to leave mojang.