How to get there? Super easy, you have to get to Puerto Juarez either from Cancun Centro ADO terminal or from a hostel you are staying in. From there you will buy a ticket to Isla Mujeres the ferry is only 15 minutes.
Where to stay? Poc Na is a popular hostel to stay you can book through Hostelworld or by emailing them directly. I recommend to call or email if its fully booked on Hostelworld because usually they put up only a limited amount of rooms on there and actually have more available if you contact them directly. There are obviously other options also, if you arrive during the day and have the energy to walk around you can find a hostel that is not listed online and stay for a bargain price. There are also many camping options at Poc Na and other locations on the island.
Where to eat? There is a great whole in the wall, Taco and other authentic Mexican little eats place right next to Poc Na, there are a lot of Vegan and Vegetarian options available also, and if you would like to splurge a little you can eat at one of the restaurants on the main street where the ferry docs (I highly recommend it, great sea food).
Party? Yes party island, beach parties at Poc Na very night.
Next… Another Mexican Island, Holbox, the main way to get around on this island is with a golf cart that can be hired in many locations.