Welcome to the forums, glad to have you onboard!
No need to apologize, in fact let me thank you for your post! I am so happy to read you bought your boy a train set, well done Dad!!!
This hobby can teach so much and you did find the best place to get started! You could have started a life long passion in your son for this hobby and that is a great thing. He will always remember Dad got him started.
I don't know much about Marklin. First thing I would want to know is if it is compatible with other HO manufactures.
You may want to switch to a more readily available and compatible brand. Here is the set I recommend for starting out with young people
http://www.amazon.com/Bachmann-Trains-Digital-Commander-Ready/dp/B000BPPNWE You could also add this track pack to expand your empire
http://www.amazon.com/Bachmann-Trai...1&keywords=ho+eztrack+pack&pebp=1418152380233 It even comes with a DVD to help you build the 4X8 layout step by step. You don't have to stick with anyone brand because most brands sold in the US are compatible with each other in the same scale that is.
I am not saying you can't build on your set rather I just don't know if you can. I do know Marklin is owned by Hornsby based in the UK and that is what concerns me about compatibility. Again I could be completely off base here.
If you would could you tell us what set you have and model number if possible. Or a picture to give us an idea of what you have.
I can almost guarantee you there will be an expert on here who can help us.
If you do decide to change brands you could always use your Marklin set for under the Christmas tree
You are on the right track

Reading is a great way to start. The more you learn before you start the better. I would take your boy to your local library. First find him some books about anything that will keep him busy while you search for model railroading books. Reading can help you narrow your focus on what you want to accomplish with your railroad.
YouTube is another great resource. This one should help you
https://www.youtube.com/watch?v=yoocLzdUAec&list=PL3B3882F24B21E496 Here is another channel
https://www.youtube.com/user/bachmanntrains they have dozen of commercial videos to search through, but they also have instructional videos too. There are many others as well.
Also look through this
http://www.lionel.com/GettingStarted/ it's about O gauge trains but most of the things apply to all scales. Here is another link from Lionel
http://www.lionel.com/ForTheHobbyist/
As one of the great model railroaders on here told me and he would tell you the same; "there are no dumb questions, the only dumb question is the one you don't ask"
Thanks for sharing your hobby with us!