What you're asking is, basically: "teach me to pixel". There is no short answer, unfortunately.
Read through this forum, soak up information, try to reproduce the techniques described in there, and generally practice practice practice. In a few weeks or months you'll see amazing progress. Be patient =)
Also shirt design has its specificities, whether pixel art or not, and a good, clear concept is a requirement. It's likely that the winning design will be decided on this aspect more than art quality. Sorry but your concept is not great atm. You need to rake your brains to come up with the best possible idea, and then roast it some more and improve it in any way you can.
I don't understand if it has to be chemistry related or not. If it is, you might take some inspiration
here (and more on google).