Someone has been filing my grandson I've had him since 2019 also found out my kids was filed I was advised to do a 1040 to claim stimulus how do I get that form

Form 1040 is the tax return form that you prepare here when you prepare a tax return.   If you enter information into the software you are preparing a Form 1040.

If someone else claimed your dependent then you will not be able to e-file.  You can prepare a tax return that claims your dependent and print, sign and mail it instead of e-filing it.   It will be processed by hand at the IRS and you will get your refund.   The IRS will sort out who can claim the child(ren) and if someone without the right to claim your grandchild did so, they will face repayment of their refund and penalties.

As for getting the stimulus payments -- you use the recovery rebate credit when you prepare the 2020 tax return to get the 1st and 2nd payments if you did not receive them yet.   The IRS issues the 3rd one later; there is nowhere on a 2020 tax return to enter anything about the 3rd stimulus check.

The stimulus check is an advance on a credit you can receive on your 2020 tax return.  If something went wrong or you did not get the stimulus check in 2020, you can get it when you file your 2020 return in early 2021

The Recovery Rebate Credit will be found in the FEDERAL REVIEW section. ( you should see "Let's make sure you got the right stimulus amount”)  If you are eligible it will end up on line 30 of your 2020 Form 1040.

Make sure that you enter ALL of the stimulus money received for the 1st and 2nd stimulus for yourself, your spouse and your children.

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-2020-tax-return

PREVIEW YOUR FORM 1040

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.