hi, good morning!
I am an angolan citezen here in africa, i love electronics and i give basic electronic lessons
sometimes i fell that i know the formula but can´t use it on practice, it is like having hands on laboratory
unfortunately our schools do not teach well, i am using multisim 8 to simulate.

i need to create a project:
what should i know first?
example: Creating a power amplifier of 90W in output, what should be the input signal power?

 

If it is an audio amplifier, the input should conform to "line in" specifications. (You can search for those.)

Unless you meant your supply power?

Sorry, it's hard to guess what you are asking.

 

You might want to find the specifications for something similar to what you want to simulate and use that to find some of the specifications for your project.

For an example, see page 12 of the document at the URL below:
http://pdf.crutchfieldonline.com/ImageBank/v20150218101000/Manuals/845/845P2500S_DL.PDF