The options depend on where you are travelling from:
From the south it will take about three hours by ferry from Surat Thani on the mainland, one and a half hours from Koh Samui, and around an hour from Ko Pha Ngan. You might wish to take an overnight ferry from Surat Thani, it leaves about eleven o' clock at night and arrives early morning, but if you use this option, get there early to grab a good mattress position, and be warned they are old cargo boats, and very basic, so you might feel happier with your own inflatable life jacket!!
If you're coming from the north, Chumphon is the place you will travel from and here you will find agents selling tickets for all kind of boats, some take as little as 90 minutes, but others will take 5 hours, so you choose according to how much you want to spend.
Another way is to get yourself on a day diving charter from Koh Samui, these are high speed boats, and you could make Koh Tao in around an hour.
However if it is comfort you are after, and a choice of departure points, such as Ko Samui, Ko Pha Ngan and Chumphon, then the Lomprayah High Speed Catamaran is probably the fastest and most comfortable way to get to Ko Tao. They run twice daily, have online booking, air conditioning, and movies.
When you get to Koh Tao there are some safety issues you need to address
You need to go snorkelling at Shark Bay, see the sunset from the Thipimarn restaurant, and generally walk everywhere, because you will see more and enjoy it more.
