Most visitors to Mull arrive by the main Oban ferry, which runs every couple of hours during the summer months, and everyday during the winter with a slightly less timetable, and the crossing is about 45 minutes. It is advisable to book your car in advance.
Another option is the ferry from Lochaline (on Morvern) to Fishnish (on Mull). This ferry crossing takes about 20minutes, and is a lot cheaper that the Oban ferry, and works on a first come/first serve basis, but you have further to drive and you also need to use the Corran Ferry. This crossing is of benefit if you are travelling from though the Fort William area.
If you are travelling from Mallaig or Ardnamurchan, then the ferry crossing from Kilchoan (on Ardnamurchan) would be your best bet, and this takes you to Tobermory (on Mull). This ferry is non-bookable. Visit Caledonian MacBrayne or call them on 01475 650000 to find out the latest costs, timetables. For enquiries and vehicle reservations call them on 08705 650000.
Remember, most of the roads on Mull are single track. Passing places are provided for on-coming traffic to pass, and if a car is behind you, then pull in to the left-hand side of the road to allow vehicles to over take safely.