A Form I-797 is issued by USCIS to communicate with applicants or petitioners regarding their immigration benefits. It is not a form you fill out – it is issued to communicate the approval or receipt of your H1B application or petition.
Here is a sample of what the Form I-797 looks like when your H1B application/petition is approved.
Sample H1B Approval Notice – I-797

Disclaimer
This sample is provided for your convenience and reference purpose only.