Maps are the main end-game mode in Path of Exile, and unlocking them are fairly easy. What you need to do is the main quest until you complete the quests offered by Lady Dialla on Merciless difficulty, which will unlock an area named The Eternal Laboratory. In this area, the map device is located, and you can insert any map into it to get portals to it.
Map drops from any level 68+ mobs in the game, and the drops are completely random like everything else that can drop. You can only get tier 1 maps from story-mode mobs, so you need to venture into the maps to get higher tier maps. You can also buy maps from poe.trade or from trade channel to get going.
The Tier 1 maps are level 68, so you should be around that level before you attempt to begin them. I would personally say that the low tier maps are far easier than Act 4 on Merciless difficulty.
Once you start doing maps, finding the Master named Zana and completing her mission will allow you to get a map device in your own hideout, eliminating the need to travel to the Eternal Laboratory every time you want to map.