As a F2P what you can do is stack up those charms/refines/cave keys and once you got the max 500 at refines/charms and 1050 at cave key. You wait a rebate and spent them there. (You get refines back and charms too, so you don't have to be at exact 500)
For sure power you can get clothes. With time you can piece together them without spending anything on them. Also you can do the binding in your home and such.

https://forum-narutoen.oasgames.com/page/show-post-29504-1.html here is the post of what gives you power

Just try to upgrade everything a bit slowly, you get resources always for playing daily so you should always be able to power-up somehow, ingots only speed up the process.

You should spend coupons on power features which you are lacking the most (the one which is least upgraded compared to others), if you are a newer player you won't go wrong with spending in Lucky Board since that event will give you items for A LOT power features

It depends on what power range you are currently at… Since you are also asking about leveling up, I’ll assume that you are on a relatively new server.

It is pretty simple. Do all your dailies. Participate in all events. Do plunder and don’t rush leveling.

Make sure to do the highest Ti for your power as Ti gives a nice power boost.

Try to go as far in ninja exam as possible. Higher exam levels = more magatma and scrolls.

Aside from that, most power comes from where you spend your coupons, so spend them wisely.

First, I recommend you to stock up power items for rebates, especially charm, and refine rebates. You don’t need to max out the rebate though. Here is a post for the rebates and best deals https://forum-narutoen.oasgames.com/page/show-post-31402-1.html

Charms you can get from the ultimate training. If they are not enough then the lucky board is your best friend until you are about 50-60k bp. Get med refines from the monthly events, group shop, and the weekly events. I don’t recommend spending coupons on med refines, since you can get them for free.

Make those rebates a priority. When you find that you have enough power items for the rebates, you can spend your coupons if there is a good event. If there is no good event, save them. Always come to the strategy section on Thursdays or Fridays and find event guides. Those will help you know which items are worth redeeming from the different events.

Outfits will also help boost your power. Redeem fragments from events and only finish them up with coupons. You can buy Anbu outfit from konoha outlet though, as this outfit is really good and you won’t find it in redeeming events anymore.

Lastly, and here is something that a lot of F2P seem to ignore:

Do Not Spend On RNG Wheels For Ninja!

Always Pick Power Over Ninja! As a F2P you will only be able to buy 1 or 2 event ninja, so think wisely about which ninja you want to use and collect their fragments slowly.

^

Basically all of this with some cheviot based on context.

Most worth your time event ninja go in the range of 15k+ coupons, though there are some cheaper ones that are good to pickup (sailor sakura being the first that comes to mind). So expect them to take awhile to get, since they will mostly be competing with clothes in most best to spend events.

Wheels are very double edge swords, since getting a good event ninja early ( around 30k bp) can launch you up in the bracket, the problem is that spending on it and not getting it could have been better spent on cloth fragments or the nin in general. So recommendation not to risk it unless you are willing to set back about 2k coupons for an early grab, since you will still get power items though normally at cost.

generally level stalling ( the process of maxing power items while min exp) in the high 70 or low 80 is most preferred, since 90% of power options will be unlocked at this time. Build as much you can here but do not expect to keep it up forever, and do remember that there is a slight advantage that having a higher lvl gives in terms of combos and such.

If you are new and need some general info on a wide array of topic I do recommend ICE*'s guides since they can help one avoid common pitfalls that could hamper a server run. For ninja's that you might want to focus on getting take a look at the unofficial tier list since it can point to which ninja are considered great for end game and give a sense of play styles you can go for.