Im assuming you are referring to a 401k plan? If so then your employer should have provided you info on your options each time you left your job. Contact the plan administrator and/or your former employers.
Your basic options will either be to 1. have them cut you a check (not recommended since you will incur taxes and early payment penalty) 2. Roll the funds into your current employers 401K 3. Roll the funds into a traditional or Roth IRA
Assuming you are relatively young, I would recommend the Roth IRA approach. You will pay taxes on the proceeds now but then you will have full control of your investments and they will grow tax free.