PI is the basic building blocks for all structures in eve. Profits can be found in all aspects of PI. Experts at the Eve University Wiki have detailed information that will help you setup your planets to extract like a pro.
Always watch your intel channels and please report reds as you see them. If attacked while carrying your command centers to planets to start extracting, please file a KOS report. This will help other players out tremendously and yourself in the future.
One main thing is that not all planets are created equal for PI. Some have multiple people extracting resources from the planet, while others have very few.
Player owned customs offices (POCO) have taxes that are charged to you and vary from system to system. Holders have agreed on a tax up to 5% to blues so that there is a uniform maximum. Some systems owners have set lower tax rates and some as low as 0%, so get out there and find your planet to setup shop on.
Talk to the corporation that owns the POCO’s to possibly negotiate a lower rate that will help you with your extraction. Finding a very low tax factory planet will help ensure you will have more profit in the long run. Factory planets are ones that are dedicated to just making P2 P3 and/or P4 items. Remember that factories do no extractions, so you will need to import the materials from your other planets.