How's that workin' out for ya'?
You make trees with a basic modeling program, or a program designed to make trees. Either way, you'll need to search for the available programs and learn how to use them. There may be some tutorials if you search.
There are a number of threads about changing the skybox, and if you search for them you will find them, I hope. I say I hope, because people who want to know how to change the skybox don't name their thread "How do I change the skybox?". Instead, they name it something like "Please help me!" and so you have to search for "Please help me!" to find the answer about how to change the skybox.
As for the sea, there are a number of ways to make a sea, probably the best way being to use the sea decals in Benges' Sprite Pack.
Best.
I'm sorry, my answers are limited. You must ask the right question.
