Pop ups & events are great places to set up & sell your merch. I would set up a booth at local shows & would stand in front of my table. I would talk to anyone that came near. This allows people to not only meet you, but also allows them to see your passion behind your brand.
Find influencers or even just people already supporting your brand. Sending free merch to influencers or people who have a solid following with your target audience is a great way to reach new people. They make a few posts wearing your merch & that will get seen by their followers.
To find influencers just search on the hashtags in your niche. Most of the time you can just send them a DM or Email.
Social media is an obvious choice here. But it's not just making an account & posting a couple photos. You need to be making content to get people wanting to rock your brand. Look at Coke & how they market their soda. They make it look like you will be having a good & happy time if you drink it. Make people feel that with your brand through your content.