Accommodations in Ios Greece are easy to find. You do not have to book in advance unless you plane to come at the very peek of the high season in end of July and beginning of August. And even then it is possible to find a room on the island even though it is a little harder during the rest of the year.
When on Ios you normally stay on one of three places. Either by the port, in the village or at Mylopotas Beach. There are other places too, like hotels at Maganari, if you like a more peaceful vacation. But the three locations above are the most popular ones. If you come to Ios Greece to party then I would recommend finding a hotel in Ios village as it is the center of the nightlife on the island.
If you stay there then you do not have to worry about a long walk to and from your hotel in the middle of the night. Staying at the beach is popular among families as you are right on the beach when you get out of your room. You are also away from the worst disturbance during the night. For budget accommodation try Far Out Camping where you can find a cheap place to stay.
You will be meet by many hotel owners when you step of the ferry at Ios port. Everyone trying to convince you of staying at their place. Take your time, look through their brochures and go with one of them if you like. They will drive you to their hotel. That is a good think especially if you plan to stay in the village as it is tough walk up the hill. You can have a look at the room before you decide if you like to stay. You will probably like it since most rooms are nice on Ios. If you do not like it then thank the owner for the ride and go hunting for a better hotel on your own. You will find lots of rooms to let so do not worry about not finding a place to stay.
There are a couple of things that you can think through regarding your hotel before going to Ios. Doing so will make sure that you get the best possible holiday in Greece. Think about what you value in your accommodation? Is it important to have an amazing view from your balcony? Do you prefer to live right in the middle of the bar street? Is the price an issue? Do you need it to have a pool or is the ocean enough? Think about what is important and you will more likely find the best hotel in Ios Greece for you and have a great vacation on the island.