To learn SAP ABAP, you need a system to practice, a book to follow and a mentor to assist you. If you can afford the price please get connected with an SAP coaching center because self-learning in ABAP although 100% attainable might be a struggle when getting a job because you don't have a certification. If you're employed in a very IT consulting firm and your company works with SAP you will request for coaching or take the self-learning route. Always remember the authentic books to learn and buy are from SAP press, the best place to get more information when you start exploring is SAP SCN and the best way to learn is by writing code and asking queries although it's going to sound foolish however ask and find a mentor. All the best.
Also, you can join SAP ABAP training courses online.