To send bitcoin to someone select the 'Trade | Send Bitcoin' menu option or click on the 'Send' tab. You need to specify the address to send to - you can do this as follows:
Add in the amount of bitcoin you want to send in the 'Amount' field. You can type an amount denominated in bitcoin (BTC) in the left entry field or the amount in your fiat currency in the right entry field.
You can add a label for your personal records in the 'Label' field.
You always get a confirmation screen before your bitcoin are actually sent. Click on the 'Send' button to see this screen. Then:
Check the details on this confirmation screen to make sure the address and amount are correct.
Press the 'Send' button to actually send the bitcoin.
When the bitcoin are sent, you get a message at the bottom of the confirmation screen. Also, the number of peers that you are connected to that have seen the transaction is shown. If there was a problem in sending your bitcoin an error message will be shown instead.