If you can't pay the taxes you owe before the due date of the original return, the balance is subject to interest and a monthly late payment penalty. There's also a penalty for not filing a tax return, so you must file it on time even if you can't pay the full balance. You should always pay in full as soon as possible to minimize additional charges. Remember that interest and penalties will continue to accrue on your tax balance until you pay in full.
You may also have to pay an opening fee or other costs associated with the payment plan. In addition, if you don't make your payments on time or don't comply with the payment plan, the IRS can take collection measures against you, such as garnishing your bank accounts and your salary. To set up a payment plan, you can apply online using the IRS online payment agreement tool, or fill out Form 9465 and mail it to the IRS.