There have been many great guides written about this game and will be more as the game changes over time. But one aspect that has helped me with this game is thinking of it as a resource allocation game.
These are the resources we need:
Character shards
Gear
Ability Materials
Credits

These are the "currencies" we can use to obtain those needed resources
Crystals
Energy (mission and cantina)
GW reset
Challenges
Keycard
Arena battles (5 free per day)


By thinking about this game as a resource management game, I have limited the number of characters I have chosen to use. As long as you do not gimp yourself with poorly chosen characters (there is a chance I did this to myself), focusing on 6 or 7 characters to level, 7 star, fully gear and get top level skills will be more beneficial than trying to upgrade 10 or 12 characters. Purple tier ability materials are the most limiting resource I have needed while playing the game (though that is relaxing some with the higher tier ability material challenge). The next most limiting resource has been character shards, followed by gear, followed by credits.

By limiting my character choices, I have gone for characters which can get me through both GW and the arena. One other way to efficiently use resources is to only focus on the light side or the dark side. I have only taken my dark side mission up through level 2 because I wanted to farm some shard on level 2 hard. Otherwise, I have not needed to go higher. In contrast, I have focused more on my light side characters so that I can open up higher light side levels to obtain the gear I need.

From a resource management perspective, the goal is to set up the most efficient crystal economy that you can. Crystals are essentially wild card currencies that can be traded for other resources. Many on the forums suggest spending 100 crystals to recharge the cantina energy 1 time and 100 crystals to recharge mission energy twice. To do this, you need to generate 200 crystals per day. A top 50 arena ranking coupled with complete GW and the energy from the daily missions will get you to this point. If you are on a server with many whales, this may be very difficult to accomplish. However, I was fortunate to be on a leaderboard with few whales. So by getting my toons to 7 star, high gear and high skilled and high level, I was able to maintain this arena ranking while other people slowly leveled up and slowly geared up many toons.

Now this only gets you to the level cap. Once there, you can think more about which toons to level and gear up. But by this point you should have a stockpile of crystals (I currently have 4.2k) by using this method.