The Sightseeing Bus Tour gives a solid introduction to Charleston and its sites. The boarding process is easy, you just need to make sure you have your proof of purchase before you get on board. Our tour was the driver talking to us but he (Neil) was excellent. He gave us a wide swath of time periods, content, and interests so there was something for everyone. The ride is about 90 minutes, you end where you started, and there is at least one opportunity to get off and stretch your legs. It is not hop on/hop off like some tours but you don't really need it.
The start point is the Best Friend Train Museum on Ann street which has rest rooms and some things to read and look at (including a replica of the "Best Friend" steam locomotive). You will likely see all of the major landmarks including Fort Sumter (from a distance but still), the market, Rainbow Row, the Battery, and others.
If you are here for multiple days, try to do this as early as possible. Some places you will want to go back to after the tour. Others, like Rainbow Row, you can sufficiently check off the list while onboard the bus. Either way, its a good way to familiarize yourself with Charleston in a manageable amount of time.