Different bus companies specialize in certain destinations. It is also possible to buy a ticket from the bus conductor, but, unless you get there very early, you are likely to end up with a standing-only ticket. Large luggage will have to be placed on top of the bus where it is difficult to keep your eye on. Also, don't wear jewelry or carry valuables. Be prepared for breakdown delays and carry your own food, water, and toilet paper.
Buses often run behind schedule except for the first one in the morning.