I downloaded SimEarth for academic purposes. The problem is that I'm not used to strategic games and I don't get this game (I don't know how to play it and what to do and so on). Is there a detailed manual on the Internet which explains all the options and symbols and which describes to me what I have to do exactly? The Manual in the Wii explains it little bit and also the symbols, but it's very hard to switch between game and manual all the time. Or is there a tutorial?
The manual might give you the controls and a basic overview, but what you may want to look at is a FAQ/Guide or a Let's Play video walkthrough.
For instance, here's a Let's Play of SimEarth:
In addition to watching someone play the game with commentary and sometimes extra information, you'll get an idea as to how to complete particular challenges and where the tough parts are.
GameFAQs is generally a reliable site for game walkthroughs, although for this game they currently only have ones targeting the PC version. The changes might be minor enough not to make a difference, so you might want to check it out. A game FAQ is generally going to have more detailed explanations of the game mechanics, and less "funny" commentary which you might not be interested in.
Here is the manual for the SNES version, which has different controls but should offer better explanation for the game.