Hi, there is an article with some info here: http://www.havanatimes.org/?p=98086
There is no Havana airport bus. In order to take public transportation you have to get out of the airport area and then find a bus stop along Boyeros street. The bus stops are not well marked. Sometimes there is a bus shelter along the side of the road, other times it’s just a line painted on a tree or pole.
If you are not experienced on how things are in Cuba, I would not suggest taking the bus to get from the airport to Havana. There is no schedule to the bus anyway. They pass every 30 to 40 minutes, but sometimes they don’t pass if the bus breaks down. I don’t know what time service ends. It probably goes almost all night.
Honestly, I suggest you just take a taxi to get to Havana. It will cost more (20 to 25cuc) but you can try to find somebody at the airport to share the ride with. Between 2 people, it will cost only about 10cuc per person to take a taxi, which is pretty cheap.
I hope this helps a bit.