I became a roommate December 18th 2008, without signing any papers but the landlord knew I was there and I was paying him $250 a month. In February he said we have to leave on March 31st. I haven't left yet. Can he just rell the cops to throw my stuff out or does he has to go through the eviction process.

You are a month to month tenant. He must follow the eviction procedure for your state if you refuse to leave on your own.

Do you really want an eviction on your record/credit report? Having such will make it nearly impossible to rent any respectable place for the next 7-10 years.

The landlord must start the eviction process on paper. Just telling you to leave is not enough. He must put this in writing. Then after you don't move out, he files for an eviction with the court. Then there is a hearing and a Judge tells you move out before such and such date or the Sheriff will put you out of the premises.