First try to find an option in your wallet that says "send" or "pay" with Bitcoin. Then you simply copy and paste your friend's Bitcoin address into your wallet's sending form ("destination address") and specify the amount. Different wallets have slightly different layouts or labels but it is all quite similar in that you will need the recipients Bitcoin address and a specific amount you would like to send (similar to Paypal, Venmo or Square cash). Once you hit "send", the amount you specified will be deducted from your balance. Once the transaction confirms (settles) on the Bitcoin network, your friend will see it become available in their balance.