Before you can get to Abrolhos, you must first go to Caravelas. Here you will find a number of tour operators authorized by IBAMA to sail to the archipelago. The cost per person for a day-trip is around R$220 (about USD 140). Check the page for Caravelas for instructions on how to get to Caravelas.
Once in Caravelas, you can only get to Abrolhos by boat. Your choices are:
Fishing boat - If you go on a fishing boat it will take around 6 hours to get there. You will be forced on a overnight stay.
Catamarã - Takes around 3h30min and you have the choice to stay overnight.
Speed boat - Around 1h30min depending on sea conditions. It is a one-day trip.
Plenty of swimming. If you dive, it is a great option (it costs around R$150 (about USD 70) for a 30min dive.
Whale Watch, Abrolhos, Brasil
Whale watch. If you go to Abrolhos on the mating season (July to November) you cannot miss a "Whale Watch" excursion. You would probably come across them on your way to Abrolhos, but it may be more interesting to go for the whale watch tour.
Plenty of mineral water and soft drinks. Avoid alcohol as you will be at sea and alcohol can be dangerous, as people normally underestimate the dangers of the sea. You do not want to spoil your holidays...
If you plan to sleep in Abrolhos you should be aware it can only be done on board of a boat. Since they are small boats, they will rock and if you're prone to sea-sickness you should take some pills or avoid an overnight stay and return to the continent in the afternoon.