Your first step is to contact the plan administrator or the human resource department of the firm. They will direct you to the service number or website to initiate the process of withdrawing money from the account. You may choose to "roll" the balance to another employer's 401(k) or your own Individual Retirement Account (IRA). The IRA can be one that you direct yourself and choose the investments or you work with another broker (or preferably an investment adviser) who will manage the account for you based on your input, your risk profile, your age, and your timeline for when you may need the proceeds.
For additional information about your options and the benefits of IRAs, feel free to review previous similar questions here on BrightScope or on any adviser's blog as this is a common question that we all have addressed.
Just contact your ex-employer and ask for the "401K Distribution Paperwork." You will be presented with a number of options on the forms. Depending on the option you select, there will be different ramifications from a tax perspective that you will want to consult with a financial planner or tax advisor on. As a general rule, I would advise you to have the money "rolled over" to an IRA, to avoid penalties and taxes.
Hope this gets you started! --Eddie