Please, Which is the best way to learn PLC and SCADA ?

Go to course. Siemens provide good PLC courses. All PLC's are based on the same principle. Once you know and understand one it will be very similar on other once. The same is with SCADA. The main principle is the same.

you need to learn both in case you want to be good at any one of them as  they complete each other as you can't make good scada system unless you understand PLc 

To have hand on experience and site maintanance.

Hands on practice of plc programming and problem solving on automated systems

Online forums on plcs

1.Through Online Courses

2.Through Industrial Training

3.Through Institute

siemens gives you good courses

Siemens gives you all the basic to the higher level so that helps alot

1.Training centers

2.Online cources.

3.online materials.