Hi...
Your textuing question, I imagine, will possably get you many different suggestions.
Texturing is a form of art which combines technical expertise with artistic ability.
Depending on your desired outcome you can use different methods.
I suggest that you use multiple terrains each with its own skins.
I also suggest that you start off using an easy method, Then up-grade your system as you learn and understand the different methods.
Using a standard "Shader" will provide you with very nice results. Combined with nice individual terrains will give you a good quality but depending on your effort the quality can vary.
First, you should have a look at the 3dgs standard shader.
But, start by getting into the 3dgs manual and do a search using "Entities" then carefully read the section about "Terrains". Take note about the "fx_terraintex" action.
Then do a search using the term "Shader Libary".
There you will find a section entitled "mtl_terraintex effect".
Read and learn about that system.
For a good tutorial that will help you with the artistic aspect, I suggest that you look at the "Mysty Mood Template" by Loopix.
When you down load their demo, you will then find their manual that explains how they enact their syatem.
They explain how to use an "Alpa Channel" which can help you greatly.
http://www.loopix-project.com/Once on their web site pick "Easy" then "Templates" and download your choice.
You will need a good paint program like "Photo Shop Adobe" or "Paint Shop Pro".
There is a lot to learn about good texturing.
As you learn these procedures, I strongly suggest that you back-up and get your present project completly under control.
Make sure the model textures are as small as possable and in the power of 2 (256x256, 512x512, 1024x1024 etc...) and that the model file sizes are as small as possable.
Another way to reduce file size with animated models is to use "Bones" animation rather the vertice animation.
Best of luck.... And have fun...