Good questions! More new owners should be asking them.
First, you need a philosophy. Are you a Running Team or Passing? or Balance? What is your Defensive Strategy?
Second, you then need players that have created builds (attributes, etc.) that match that strategy.
Third, you need active agents to log in often to spend SP in a manner that is consistent with what your game plan is.
Fourth, you need to build a playbook. Start with Offense. Initially, limit yourself to 5 plays of each type as that's the minimum. The default playbook has 15 or so. This way you'll call the same play very frequently during a game. You need to evaluate if the play is being run successfully. That means watching how the players move and interact. What you want to do is either play to your strength if you went all run or all pass, or give your team options if you want balance. Example: Out of the 2WR sets, you want to have running plays that go inside and outside. If you're a balanced team, then you also want passes out of the 2WR sets.
For Defense, again cut down the number of plays in the playbook. Have one or two blitzes for each WR set, a base man D and then some situational plays, like Inside Run Blitz which fires off of a different tactics setting than other blitzes. Again, you want a small playbook so that you'll run the same plays over and over. You need to evaluate your team and whether the play works for you or not.
Important: Change out the Defensive plays in the standard playbook. You'll get killed on outside runs with GL Gaps Fire crap. Play a Man Base D or investigate what else works, but get rid of those default plays fast.
The blue dots on the play indicate frequency. If you have 5 offensive plays with 5 blue dots each, then there is a 5/25 chance each play is called (or 1/5 obviously). If you have 5 offensive plays and one play has 5 dots and 4 plays have one dot, then there is a 5/9 chance that the first play is called and a 1/9 chance the other plays are called.
Fifth, tactics settings can be daunting but start off with a base setting and then review every game against those settings. Were you calling running plays when you wanted them or were passes called? Did you Blitz in the right situation or not? Etc. Once you've played a bunch of games with the same settings and understand them, start tweaking some settings and then see if it has the desired outcome.
Finally, if you have the Flex play scrims. Lots of scrims will quickly accelerate your learning curve.
This is just a start upon a long journey.
Have fun! (and bash Corndog over how Chemistry is implemented even though you have no idea what I'm talking about

)