If it's not booting and is stuck on the color GPU test screen (with the lightning bolt), it could be old firmware from an out of date NOOBS card.
If it is booting and you are getting a low voltage warning, then your Pi3B+ is receiving less than 4.63V, and the PSU is likely to blame. If the PSU has a separate micro USB cable, you could try a different cable (the ones for charging smart phones or tablets are usually better).
We need more info about the power supply. Got a link or specs? Unfortunately there are some crappy "power supplies" being sold that don't output what they claim. They are usually repurposed phone chargers with poor voltage regulation.
There is an official power supply which works very well. Click the green BUY tag at the top of the page.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?