The question may sound silly but it deserves to be answered.
Some people would wonder how “difficult” it is to go from one place to another and the answer is no, definitely.
You have to remember that everything is so easy in these countries, and everyone comes to you when you look a bit disorientated. And the people that will most likely give you a hand, are not the most altruistic people : the tuk tuk drivers. As you walk passed them, they won’t miss a chance to remember you that you’ve got a heavy bag and that it would be best carried on their tuk tuk-machines rather than your back. Somehow, they care about you health. Good point. The simple fact that you are caucasian will make the following question come to your face many times : “tuk tuk ?”
Although sometimes it will be useful to use the services provided by theses “not so quiet three-wheels” machines when your accommodation’s location is too far for what you’re willing to walk, loaded like a donkey. This is basically the first and/or the last step for going to some other place : going to a terminal from your accommodation and the other way around. So you don’t need to worry too much because the tuk tuk drivers really know how to deal with tourists. They know where to pick them up and where to drop them if you don’t know yourself where you’re going.
There are different ways to go from one place to another though.
The accomodation that takes care of everything
The easiest way is of course to use the services provided by the accommodation you’re staying at. Most of them provide this service. They will arrange you the transportation from their place to your destination, tuk tuk or taxi included if you need to reach a bus or train terminal in between.
But this solution will cost you a bit more of money because they will indirectly charge you for the services provided even if they won’t mention it explicitly (usually the extra charge is between 20 and 50 bahts than if you’d had handled the whole process by yourself)
You manage the whole process on your own
This is (usually) the most economical solution. If you can walk until the bus terminal, you do it, shutting off at the same time a large amount of tuk tuk drivers that you will run into on your way. Then, you buy a bus or minivan ticket directly at the counter or inside the bus. There is no rule, and no need to know how to speak Thai as long as you can pronounce the name of the place you want to go to.
If you have the choice between a regular bus and a minivan and they’re the same price, I would suggest to hop on a minivan. They are in my sense, more comfortable and faster, a bit less reassuring though (see this article).
So this is pretty much very simple to go from one place to another, you can even do it easily on short notice.
Same thing for boats or trains, but be careful though when you’re going to a highly touristic destination. It could be wise to anticipate so you don’t get screwed after you checked out but it shouldn’t be a major issue though. I’ve never been stuck somewhere because a bus or a boat was full.
Obviously, if you’re planning on booking a flight, you should do it as long in advance as you can.