A crash on start-up usually indicates a missing master file, or a master file that is loading late.
First, download and install https://code.google.com/p/better-oblivion-sorting-software/downloads/list to sort your mods if you haven't done so already. Check the BOSS log and review any error messages it returns; in particular, look for messages regarding missing master files, incompatibilities with other mods that you're using, or plug-ins that shouldn't be used with other plug-ins from the same mod. If you're missing a needed master, download and install it. If you have two mods that are incompatible with each other, choose one and disable the other. Disable any files that shouldn't be running with others. Rerun BOSS to properly sort everything once you've gotten it all straightened out.
If that doesn't fix the problem, review the documentation for your mods and make sure you have all files necessary for them to run. If you need files, get them and install them.
If it still crashes, re-verify your local files with Steam (right-click on Skyrim in your Steam folder, select Properties from the bottom of the menu, click the Local Content tab in the window and then click the "Verify Integrity of Game Cache" button). Delete your Skyrim.ini and SkyrimPrefs.ini files, located in the My Documents\My Games\Skyrim folder.
Finally, if nothing else works, disable all files except the main game files and any DLCs you may have, and try to start a brand new game. If it still crashes, you have a problem with your vanilla installation and you'll need to fix that before you can do anything else.