Before creating Facebook App you need to get admin access to page in Facebook which has lead form. If you are not an admin of cause.
To grant another Facebook account admin rights to Page go to Setting of the Page, then to Roles.
Enter email of Facebook account that you want to grant, select Administrator role and Save.
(!) You need admin rights on Page to get ID of this Page and ID of Lead Forms.
To create Facebook App visit https://developers.facebook.com/ (you should be already logged in Facebook).
Go to My Apps -> Add a New App. Then you need select Website type.
Enter the name of your App and click Create New Facebook App ID
After it appears popup. You can select the category of your App, but it does not matter for vTiger integration. Click Create App ID
After App will be created you need to put Site URL
It is URL of your vTiger CRM. You can fine it in Wizard during Facebook to Lead installation.
Click on Next, scroll down and see Finish massage. That is all.
Then go to created App and find there App ID and App Secret that you need to enter in Facebook to Lead Installation Wizard.