Read this thread if you didn't already: http://minetest.net/forum/viewtopic.php?id=3341
and/or try to install minetest again.
If you did install minetest correctly and still have the same issue, we'll need more info:
- Close minetest. Delete the debug.txt file in your home directory. Open minetest. Try to create a world to reproduce the error. Move the mouse a bit (sometimes my created worlds don't appear until I do that). Close minetest. Open the debug.txt (doubleclick it). Look for "ERROR". Copy that part and paste it here in a post
- Other info that might help is your OS (windows/linux), installed version of minetest (0.4.3 for example) and the installed mods.
- Can you create a game under the 'advanced' tab?