From the Send & Receive tab you can create transactions and invoices. You can also create transactions with messages, encrypted messages and/or mosaics (assets.) You can read more about mosaics later in this guide.
To send, all that's required is to enter the recipient's address and the amount you want to send. Then enter your password and hit Send. Listen for the "ding" that confirms it.
Transactions usually require a small fee. The NanoWallet shows the fee and automatically adds it to the transaction. There is a small additional fee for sending a message with your transaction.
Some exchanges require you to include an identification message when you send XEM from your wallet to your exchange. If so, the exchange will give you a code to include in the message field when you send. Double check to see whether the exchange you are using requires this.
If you make a mistake and forget to include the message you may lose the XEM you sent, although this is rare and in most cases it's possible to recover them with help from the exchange's customer service staff.
Note that this only applies when sending to an exchange, not when receiving.