What particular problem are you having with invoices ( i.e. creating the invoice, sending the invoice, printing, adding a logo and so on). However, I have some useful articles and video tutorials that will get you better acquainted with using invoices.
For Creating an Invoice,
Go to the +New icon.
Under Customer, select Invoice.
Enter the customer information and add the product and service provided.
Add the quantity and rate, the amount will automatically calculate.
Add any additional information like sales tax rate, if applicable.
Save the invoice.
If you'll like to send the invoice to the customer via email, select the open Save and Send. Once that option is selected, you can see a preview of the email before sending it. On this same screen, you have the option of Printing the invoice as well.
For more information on what to include in an invoice, this article is very beneficial. I'm here if you have additional questions on Invoices. Enjoy your day!