Rolling over a pre-tax employer-sponsored retirement plan to a traditional IRA is not a taxable event. However, a mandatory federal income tax withholding would apply on the distribution unless you have your former employer's plan send the money directly to your new IRA custodian — or send you a check made payable to your new account with an IRA custodian. Contact your IRA custodian if you are unsure of where the money should be sent. If you don't, your former employer must withhold 20% for federal income taxes.
When the money is sent directly to you, you have 60 days to put the funds, including an amount equal to the 20% withheld for federal income taxes, into an IRA, or it will count as a distribution. This could subject you to federal income taxes and an additional 10% federal tax if you're under the age of 59½ and no exception applies.
You can get started with a rollover by contacting the plan administrator. A rollover IRA isn't right for everybody. Consider all your choices in consultation with your tax and financial advisors and decide whether rolling over may be the right choice for you and your retirement goals.