I would suggest painting up a few other models first to get a bit more of a feel for painting, you are right it is a huge and centrepiece model. If you don't have much else, at least paint the skink crew before the rest. And since all of the LM army is effectively lizards with scales and skin, you can experiment with the scheme for your stegadon on a smaller model to see how it looks before tackling him.
I think one of the most important things you learn as you develop painting skills is confidence though, same with converting. When you start out, you are afraid to make a mistake so you are afraid to try. I went through that with lots of early models not getting too much paint. Just remember, it is easy enough to paint over mistakes, or if they are painted on thick it can be stripped back and started again. Don't be afraid to get it messy and try new things, you will never get good at the more advanced techniques like highlighting if you stick to drybrushing. Sure the first few tries might look a bit off, but just keep practicing, post pics of what you have done and get feedback.
As to the red stegadon, there are several ways to get red and many different shades of red. What kind of red are you looking for? How light or how dark do you want it? You can go for a fairly dark and neutral red/brown look, or go pure red and highlight up to a blood red look, or even make it really bright and highlight up to orange/yellow as I have done with my carnosaur and skink scales.
As a general basic guide, you should aim to have 3 tones of the colour and the middle one will be the look you are trying to achieve. The darkest will be the base coat which you apply first. Then a mixture of the dark and middle for a coat over almost all of it, leave the deeper areas alone. Then the straight middle colour leaving a little bit less alone. Mixture of middle and light, and finally light. The 3 colours might be scab red --> blood red --> blazing orange, or scorched brown --> scab red --> blood red, or whatever tone and look you are going for.
Also the thickness or thinness of the lightest colour will change how the model looks, and applying washes at various stages helps to smooth your highlights and add depth where you may have missed.
As an example, here is my WiP carnosaur: (Looks like Sammy disabled url tags so you will have to highlight it)
http://i218.photobucket.com/albums/cc80/strewart/Lizardmen/carnosaur.jpg
It started off with scab red as a base coat, then had 50/50 scab/blood red as a first thick highlight followed by straight blood red. Then it got a coat or two of red ink which is why it is shiny in the red areas. Then a highlight of mechrite orange, and finally a very light highlight of bad moon yellow. Now it needs a bit more ink/wash to blend it all a bit better. In hindsight, I should have done more between blood red and orange, but there ya go.
Hope that helps.
, what colour's and what order do you suggest? Thanks!