There are two ways you can set the amount payable upon submitting a form.
Set amount, the same each time
Variable amount, based on a Total Field.
Set amount:
If you want to set a payment amount that is always the same for every person that submits a form, start by going to Application types > select 'Edit' on the Type you wish to edit.
Click the Payment required check box. This will reveal a few more options.
Then, in Payment Amount, enter the static amount that you wish to charge whenever someone submits a form.
If you want to take the money on submission, you will need to select Enable Paypal, select this and then add the paypal username.
Variable amounts:
As above, go to the Type, and select Payment required.
Now, select 'Payment Amount Element' - this asks you to select which Field in the form you wish to use as the payment amount. This will normally be a Total field where any amounts have been added up in the form.
If you want to take the money on submission, you will need to select Enable Paypal, select this and then add the paypal username.
Then, whenever a form is submitted, when the person completing the form hits 'submit' they will be taken to Paypal, and asked to pay the amount above, either a fixed amount or a variable one based on what has been selected in a form.