Welllll, this is not how you sell tshirts son.
Consider changing your website's design. If you do the same thing that everyone else is doing, you won't get any conversions. Tbh, your website's deign is too plain. The tshirt's color which is white gets mixed with your website's background color, white. Who ever designed that website needs to quit his job.
If you aren't moving with the trend, your products won't sell. Your designs are just plain and simple and won't stand out amongst the others.
Who wear tshirts the most? Teenagers. Whom should you target? Teenagers. How do you target them? Internet.
For example, a plain tshirt which says "Feminist" can beat all of your designs, why? coz that's the trend now.
Where to get ideas from? Use brains and internet. These two links will help you. Google trends and buzzfeed.
How to promote your product?
Facebook:
Start with Facebook Ads.
Choose a few products of one niche, for example if it is related to Feminism or some quote related to beards, you know which gender and age group to target.
Target the geo locations with big universities, or the one's which are rich. There's no use of targetting one whole state in the US, not everyone is interested in
buying tshirts.
If you want to target some spefici music genre or musician you know what kinda people you should target too and what age group. If there's a concert of some big star in a few weeks, target the artist, make a design, target the audience.
Another example of targetting people is, if you have some designs related to Weightlifting/Fitness, target the people who like and follow fitness models and not the people who have an interest in fitness, there's a difference here.
Create a facebook audience through your facebook page.
Create Ads every once in a while, throw offers ocassionally.
Learn about Facebook CPC, learn everything there is to know about how facebook ads work and efficiently spending on it.
Reddit:
Do you have any tshirts related to coding or a famous meme?
Target the sub reddits of that niche. Reddit ads are neglected most of the time but they can convert very well too.
Instagram:
Buy an account from someone here which has atleast a 1000 followers and use it as your official page.
Promote your products through influencers. Run ads here too.
Even after doing all this i can't guarantee you conversions because of the website design and the quality of mockups you have right now.
People see those boring mockups everyday. Even if you are posting ads, their brain won't even pick your ad. It's a straight r/whoosh.
How do you come up with something different? I don't know put brains to work. The ideas are LIMITLESS.
Do you know which shopify sites rank the highest on alexa? Check those out.
Designs aren't the hype these days, Most of the top selling tees are just simple text. Choosing the right Font and colors is the crucial part. Looking at the way you chose the colors for your website, maybe you might fail at this part too.
Think logically, what people would like. What would want them to buy from YOUR website. It shouldn't look like a new website even though it is one. Make people feel comfy when they visit your website, it should look and feel like it is a website made only for a selected few and not for everyone. Your design should make people spend that extra $10 for the quality of your presentation.
I have been in the fashion industry for past 3 years, even thou its my family business i wouldn't say i'm a professional in this field. But, i do have a lot of insights on what make a brand stand out.
Good luck OP, i hope you make some changes and make your website one of the best.