Painting my room

Hi

I have to paint my room again, I'll be using the same color. I wanted to know how to prepare the paint starting from opening the can of paint.

Should i mix water with the paint? If yes how much?

Also any tips about buying a can of paint?

What about using rollers? Watching videos on youtube - they had a special type of container for keeping the paint when using the roller. Is there any special container being sold in Mauritius for using rollers?

Thank you.