In my experience when it comes to stuff like this, it's worth paying to have it professionally designed. You should aim to sell merch that people want to buy.
Hard thing is selling. You can have awesome fabric, cool design, and all that customers will like in your merch, but if you suck in advertising and creating a brand name, you will lose competition very quickly. My advice will be to buy or find) some cool mock-ups for your brand to first present look. It can be 3d mockups, flat mockups model mockups, etc. - "You are judged by appearances at first but by your mind later on"
I'd recommend getting yourself an HTV (Heat Transfer Vinyl) cutting plotter if you're diving into the world of merch. It's a game-changer! With this nifty tool, you can create custom designs and transfer them onto various items like t-shirts, bags, and more. The cool thing is, you can make stuff in low quantities without a ton of risk. It's a perfect way to kickstart your merch-making journey!