you shoud buy Mount Myōboku (妙木山, Myōbokuzan, Literally meaning: Mysterious/Exquisite Tree Mountain), also known as the "Mountain of Bewilderment" (迷いの山, Mayoi no Yama) and the "Land of Toads" (蝦蟇の国, Gama no Kuni) gifts. they are the best what you can buy.

https://forum-narutoen.oasgames.com/page/show-post-25831-1.html

ohh sorry and I'm just a beginner lvl25

Save all those coupons, spend them either during lucky board or mission general mobilization event and redeem either ninja you would want to have in the future, medium refines or charms. Even though you will unlock charms at lvl80, it's a really good investment. Never redeem common thread/ low refine, you get past that level really quickly.

what about buying magatamas? lvl 2 magatamas i have 900 coupon

Do not buy Lv2 Magatamas or any kind of magatamas .. You can get it for free by just playing Ninja Exams and resetting and sweeping it every day .. You c*so get Magatamas in participating in Nine Tails Invasion and other weekly events .. Also, in some events, like the Lucky Board, you can get magatamas in there while getting some redeem points too to redeem some ninja fragments or power-up items, Lucky Star Wheel event also have Lv4 or Lv5 Magatamas to redeem sometimes .. So just like other said before me, just keep your coupons and focus on buying some stuffs that adds power, and also for buying ninja frags that you need (not want) to form a decent team for you to use in your battles ..