The Cobra is a cheap and versatile ship, ideal for first attempts at piracy. It has enough module slots to fit everything you need to start releaving hapless victims from their cargo.
It's also sold in Harma's main station, Gabriel Enterprise. It's shipyard sells everything you require to get started.
I recommend fitting a decent Interdictor in the single g3rd Grade slot. The 4th grade slots are best for a Shield and Cargo module. That gives you decent defenses for when things go south, and 16 cargo spaces to work with. The more you have the more limpets you can bring and loot you can scoop. Since staying alive and grabbing loot are our bottom lines, using the best slots for these modules makes a lot of sense.
Elite Dangerous' way of communicating "drop your cargo or die" is to remove a ships shields. I recommend filling your medium hardpoints with gimballed cannons and your small hardpoints with laser turrets. That way your ship will automatically deplete a targets shields without damaging it's hull too badly. The powerfull cannons can assist with this job or, once shields are down, destroy a targets cargo hatch. It's also usefull to attack ships when they fight back, or finish off targets that don't pay up.