Can we understand machine learning by learning online completely ? If yes how ?

Answer 1

Yes if you are really determined to do so, you just have to know where to search. Lots of coding, watching and reading will be needed, but it's surely possible.

Answer 2

A strong background in mathematics can also help. Knowledge management concepts may also be required.

Answer 3

You should take a look at the framework Keras for python. This will give you a good basic with lots of tutorials to build neural nets and other stuff. SciKit-Learn is a more powerfull but also more complex alternative.
And here are some "keywords" you should know well for further understanding:
- Activation Function
- Error Function
- Backpropagation algorithm
- Optimizer
- Regularization
- technical neuron
- hidden layers
Some useful python libraries:
- numpy
- matplotlib
- pyplot
I would also recommend to get the basics for data-science. In my last projects I had to learn that data knowledge can be extremely powerful!
If you have any further questions feel free to ask me :)