The first thing to do is do assess whether your conversion rate is really that low. E-commerce conversion rates are around 5% for fashion and apparel. Considering you are a startup, then it's quite reasonable that you'll be getting a far lower conversion rate than the 5% average. Here's more info on e-commerce conversion rates - https://www.smartinsights.com/ecommerce/ecommerce-analytics/ecommerce-conversion-rates/ and https://www.invespcro.com/blog/the-average-website-conversion-rate-by-industry/

If your conversion rate is really low, then you need to determine the cause. Is it pricing, product offering, targeted customers, etc.?

If your price is higher than your competitors, then try to justify that higher price before dropping it to match (sure fire way to run out of business). You don't want to race to the bottom of the price pile as existing, entrenched competitors can undercut you. So, promote your benefits (faster turnaround time, quality products, impeccable customer service, etc.). Sure, there will be customers purely shopping on price, but there will be those who are willing to pay more with adequate justification.

Maybe you are targeting the wrong customers? Take a look at your current and past customers and see if there's a particular profile that the majority of those customers share. Then focus your marketing and website on those customers.

It sounds like you are doing custom apparel and other decorated goods, such as promo items. Most customers (typically businesses) looking for promo items and corporate gifts typically like to deal with as few vendors as possible. So, you can start with your existing t-shirt customers and ask them what other products they purchase or would like to purchase. It's a lot easier to sell to existing customers than to acquire new ones.

Good luck!