Never buy clothes.

You can save your coupons early for one of 2 things, either getting a decent ninja- in which case you want to continue to save up till you have about 20k Coupons, this will take a few months depending on wither you purchase monthly card or not.
If you decide you have the patience for this option, you're looking for a Ninja that is strong & will remain strong for the rest of your journey, that means something like Angel Konan- though the decision is honestly up to you. You want to wait till there is a coupon rebate "Fukurokumaru deals" usually, that will give you a ninja just for spending 20k coupons, on top of the ninja you will also buy.

The 2nd option is purchasing power, which means saving up for certain events you can spend in that give points for you to exchange for rewards while also giving you power items as you spend, the best event for this is Lucky Board which is available roughly at the start of every new month.

Never buy anything out of the "Shop", the Groceries Storage Room is best & that requires you to do your daily events to get sun & moon..

You don't buy clothes because there's a max amount you can get & it's easy to reach that cap by doing your weekly SA, it's literally a complete waste of coupons to buy Clothes.

Outfits are completely different thing, which I'm not talking about.

This isn't the case when you're new to the game, chances are when you're first starting the first thing you notice as Clothes are going to be Common/Medium & Advanced Clothes, when someone says "Buy clothes" someone new can very easily be confused by this & mistaking buy these. Until they're renamed to something otherwise, I will continue to call outfits, outfits & C/M/A clothes, clothes.

That said, saving for outfits early isn't always the best option either as you can get a lot of power from power items early on, Lucky board is a great example of this(Though I don't think it's that great, but it's just an easy way to dump coupons without really having to think)- once you get around 60-70k power is when I'd stop focusing on Lucky Board/power items & spend on outfits.
Though picking up a decent ninja early is also really beneficial if you can be patient for it & is what I'd do if I was starting again but I have the luxury of knowing what is useful & what isn't.