MythTV has a lot of different components and dependancies. It can can hours, if not days to get the right version of each thing MythTV depends on all installed and working together.
It doesn't have to be that way. Some people in the MythTV community have done a great job creating easy to install MythTV Linux distributions that are purpose built for running MythTV.
Generally the downside to using a MythTV distribution is that they format your hard drive and start from scratch. If you want to install MythTV on an existing system without losing your other Linux functionality and files, then a MythTV distribution isn't the route for you. However, if you have a PC you want to fully dedicate to MythTV, then these are a great option.
There are two popular MythTV Linux distributions, and many less popular ones. I suggest you stick with these two unless you have a good reason and prior MythTV experience.
The two most popular MythTV distributions, which I suggest you start with if you are new to MythTV are:
Primary MythTV Distributions
In their words, it provides "an Ubuntu based MythTV distribution that is easy to configure, maintain, and use." Mythbuntu is the newest of the now three major distributions. Visit the official site.
If you like bleeding edge and constant releases, Mythbuntu's for you. If you're a linux geek and want this as a hobby, KnoppMyth may be your speed, and I don't mean that in a bad way. MythDora would be what would I suggest for a "just make it work" distribution.
If you have the time, I suggest you try each to see which one works better for you. I don't think one's really better than the other. Each one goes after a slightly different type of user.
If you have hardware problem with one, another may work for you.
Also, at 20 minutes to install KnoppMyth, and MythDora less than an hour, it's not that big of a deal to try them all if this is a system that you plan to use for a while.