When an invoice is first created, it will always be in Draft status.
When you've confirmed all the information within the invoice, change the status to "Pending". You can only approve invoices from the Invoice List if they have the status "Pending".
You can (depending on user privilege) approve the invoice without setting it to "Pending" status by editing the invoice.
To approve invoices from the Invoice List:
You also have the option to approve an invoice when you're editing it.