Game Objective & System
Before running a game, you need to make some important decisions. A lot of times people start by picking a system, then they build a campaign idea, draw up some maps, pick some monsters, and go for it. In many ways this is just fine, but for modern games, I find that there needs to be a little more on the back end.
Back in ye-olde-days, of RPGs, games were about killing things, breaking traps, and getting XP. This is pretty clear when you look at how old systems were set up based on kills and treasure gained. As RPGs expanded and more systems gained popularity, there was a trend toward super mechanics heavy game systems (D&D 2nd Edition, Shadow Run, etc.) But over the course of time, systems have been winnowed down considerably, focusing less on XP and more on the game play experience. Now there are super lightweight systems like Unisystem, GURPS, Savage Worlds, etc. that are designed to work (with minor adjustments) in nearly any setting. The emphasis is on the story telling and role play. Classless systems allow for more customized characters, and downplay min-maxing.
Anyway, all this is to say that before picking what system you want to use to run your game, you should probably take a step back and consider what kind of game you want to play.
I call this, "The Game Objective."
The reasons behind why you want to run your game are going to change the type of game you run. Playing a game to decompress from the week, playing a game to experience something new, and playing a game to exert some control over life in a chaotic unsure world, will all yield vastly different games. And you can have multiple goals! Before starting campaign planning, I suggest you ask yourself these questions:
What do you hope to get out of the game?
Why do you want to run this game?
What do you hope your players will get out of playing?
I want to explore this awesome world I made up.
I need a creative outlet so that I don't murder my co-workers.
I want them to bask in my glorious creativity.
In this case, maybe I don't want to play a hack and slash campaign. I probably want to run a game that involves a lot of travel. Immediately I start thinking about collection quests. Maybe characters need to collect items from around the world? Maybe there's a machine that needs to be built? I probably need a system that has a nice mix of combat and RP.
I want to hang out with my friends.
I want everyone to have a really good time!
So, maybe running a super serious, dark, twisted game is not best.... Maybe I should focus on a light system that encourages wacky behavior! Maybe 7th Sea would be good for giving players lots of flexibility! And who doesn't love pirates? THE BAD GUYS CAN BE NINJAS!
In summary, before settling on a system, explore your goals and then do some research. There are lots of great systems out there that are fun and easy to learn. Playing a new system also puts all of your players on a more even footing, that way you don't have to worry about players obsessing over being the super best, or using the rules to their advantage in an un-fun way. (Unless you're into playing by the rules, in which case, check out Shadow Run 5th edition - it's a super fun rules-heavy system. ^_^)