Grand Arena Guide?

Here are some thoughts:

1. Decide how much time you want to spend on GAC.    If the answer is very little, set the strongest defense possible.   See what score they get, and then respond with whatever you have.    If the answer is that your priority is winning, then review your opponents history before setting defenses and save enough teams to do some damage.
2. If you love to attack and get big scores, hold out your best teams for offense, and attack first.   Some folks don't like to invest the time, might attack once and stop.   This happens more often in 3v3 because it is an uncomfortable time suck.    Hard to say if it works like that with new players.
3. Datacrons are now the main differentiator in Kyber, I imagine it is like that elsewhere.     Maybe not advice you can use right now, but someday.    One datacron on the right team, early in a GAC season, can sometimes carry you for some wins.   (either because it is OP, or people don't know how to handle, or because it is early and people don't have the new DCs yet)
4. CG expects your win /loss to be 50/50.    Win more you go up and lose you go down.     So, don't get frustrated when you lose some matches - you are expected to run into bigger rosters and lose. 
5. You can always post your roster.    Folks might actually tell you specifically what to use. 
6. If PHX is your best team, check your opponents roster.   Do they have a counter?   If not, consider putting on D.   (this assumes you have the time to waste on a bunch of prep)
7. Like others have said, once the round start, check the counters guide.   Don't be afraid to google.   There is a ton of content out there - who to attack first, synergy, which specials and when.... it's probably all there if you have the time. 

Enjoy the grand arena!