The following sites are starting places to get ideas for your own bills.
Find something that is of interest to you. Your bill needs to fix a problem in society.
You will spend a lot of time becoming an expert on the problem your bill seeks to address , so find something that you will be willing to spend time on.
Ideas for mock congress

Bill Idea Page

23 Laws We Should Adopt in the United States

Once you have decided upon your bill you must research it and then decide upon the exact wording of what you address as the problem and the SPECIFIC ACTIONS your bill REQUIRES to happen.

When you have done this READ

THE PRINCETON MODEL CONGRESS bill requirements and use  their
 template to write your bill (Change the Princton heading to  US SENATE