All of the money that you contributed is 100% your money. If you are 100% vested, then that means that all of the money that the company contributed is yours too. All you have to do is open an IRA account with a company like Fidelity, Vanguard, T Rowe Price, and transfer the money. It's very easy to do all you need is the account numbers and a few signatures.
Or you can keep the money right where it is and nothing happens. The account is still yours, and it is separate from the company and they have nothing to do with that account; you can leave it there indefinitely. Another option is to transfer that 401k to your new company (it's called a rollover). The process is exactly the same as moving it into an IRA.
If you were only 50% vested, then the only money you wouldn't keep would be the company contribution. All of the money you contributed would still be yours. Only 50% of the company contribution would not be accessible to you.