Varadero: trip to to Havana?

Question

I am a Canadian traveling to Varadero in about a month. I would like to take a whole day to visit my friend who will be in Havana at the same time. My questions are:

1) How do trips from Varadero to Havana work in Cuba?

2) Is there a way of planning/booking a bus or tour before arriving in Cuba?

3) Is there a tour involved in every trip, or can it be simply a bus ride?

4) How much will this cost, and how would I pay?