Usually when someone sends you friend request, you will get a notification. You can find this notification in your menu bar of faceook app. As for example, i have got a friend request and 2 options are with it.
"Confirm" or "Delete". If you want to accept the request then click on "Confirm" button and if you do not want this then you should click on "Delete" button.
When you click on confirm, then you will be friend with each other as shown in below figure.