1. Power before ninja: Don't spend coupon or event point for sake of a shiny new ninja, instead use them for stuff that will give you power. There are, of course, a few exceptions, but if you are asking these type of questions, you probably aren't ready for that.
2. Never buy stuff in the shop, NEVER. All coupon should be spent on event. Without making things too complicated, I'm just gonna tell you spend it ALL on lucky board. And use exchange pts for charm pack and medium refine. (the rune cave used to be a good way to spend too, during rebate, but getting key off the wishing tree gives a better key to coupon ratio, so just get keys there and only use keys during rune rebate)
3. Even over focus: Everything should be spread as evenly as possible. Sometimes it is better to have left over * than trying to use them all. Magatama, for example, should all be lv 3&4, or 4&5 or 5&6, never 3 or more different level equipped and never an empty slot. There is a tiny exception for this but it doesn't do much and I don't want to confuse you. Same with summon rune, even level in all 3 chars. Rainbow magatama(if you can equip any), even across all slots. The ONLY exception is refine, in which you should focus on one piece of gear at a time during wish credit event to maximize its effect.
4. Do everything, do convoy, TI, SA etc, join every fox, sage, GNW, quiz etc. Now matter how bad you might be at it. Participation reward is better than nothing.
5. Spend your item in event only: refine during refine wishing credit (the rebate can be used to some extend, but that gets complicated), threads during battle armor rebate, cave key during rune cave rebate, seals during specific treasure rebate(like the one last week, NOT the one from 3-4 week ago), charm during charms rebate, summon during summon rebate. Also, only use EXACT amounts to get a certain tier of rebate then stop--if have 127 cave key, for example, do 50 then STOP, wait for the next one (where you hopefully will have 200, the next tier). For refine wishing credit one, you should only do it if you have enough medium to push to lv 7 directly, that's about 60-100. Stay on the safe side and collect 100 and get one piece of gear to 7 at a time. If you have more than >100 left over, do another. Else, stop.
There are many many things that can improve your power. Those are just some of them. But they are the simplest guidelines and I don't want to information overload you. As you progress, you will probably learn the minute fine points that can improve your power further.