I have L1 visa and I am employed by US company which is the form I have to fill in to pay the taxes?

How long have you been in the U.S.

Are you married?

What country are you from?

You are nonresident alien. But for tax purpose, you may be resident alien. It depends upon the number of days in the U.S. If your stay is over 183 days, you file as resident alien. If your stay is between 31 days and 183 days, you may have option.

Non resident alien will file Form 1040NR and a resident alien will file Form 1040EZ/1040A/1040.
For more information read IRS Publication 519: U.S. Tax Guide for Aliens.
Internal Revenue Service

Jlorea:

MukatA is probably right that you will have to file as a non-resident alien for 2007. For 2008 and beyond, you will file as a resident alien.

You ARE liable for Social Security and Medicare taxes from your first day IF you intend to stay beyond 183 days.

If you are married, you can file jointly with your spouse and both of you CHOOSE to be treated as resident aliens for all of 2007. In doing so, you must claim ALL of your world-wide income.