I think it's good to read the "Reference Manuals" before starting with the "Robot Tutorial", but which, the first three (Robot DNA, Concepts, Simulation Options) or all
I suggest you start with Purple Youko's tutorial, read down to and use the simplebot link. If you read through simplebot and perhaps try out some of the versions then you should get a pretty good idea of what's going on. (You can just read through it all though, it is a very good step by step introduction to some of the basic ideas behind writing a bot.)
I have looked again but haven't read through all of the simplebot stuff or tested it; I'm afraid some of the information is out of date now but it looked a pretty good introduction to genes and the stack though.
Following that read the basic bot tutorial, you'll have a good idea of how genes work by then and the tutorial provides a link to learn about the 'stack' if you need more information.
The reference manuals are just a bunch of facts and figures, I don't think it will help you understand how bots work as well as those two tutorials.
If you find problems with PY's tutorial just pop back here and I'll sort it out, values being different and some of the old bots he's suggests trying it against are some you might find. (It is an old tutorial after all!)