You can read the Userguide here.
Here are a few tips to help get you started...
1. The first thing you should do is fill out your "Preferences". These are used to setup and "fast-fill" your invoices
2. Next you should add some products and/or services you commonly offer. These too are used to fast-fill your invoices
3. You can add new clients and customers to the Contacts app when you create a "New Invoice". After that you can use the Contacts app to fast-filled your invoices for them.