You can either go to Invoice module, then click on Invoices on the left menu or you can click on the QuickAdd menu and click on Invoice
Once in the Create Invoice page, click to add a Customer if it does not exist, if the Customer records are there, the select.
Then make sure Invoice Date (defaulted to current date) and Due Date are correct
Move to the Item section where you can add Product and Service under Item, Description, input Unit Price and Quantity
Please click Save after you have added all the items needed
You can export to pdf, excel or you can send the invoice via email
If you are getting paid via Bank Transfer/ACH or Credit Card then you need to set up the payments by clicking on the Connected Apps (Settings > Connect Apps)