Hey Alex!
Congratulations on starting your journey. We all have to start somewhere and it sounds like you are doing things the right way.
I work with a couple of builders who have been been a part of my community for decades. I may not be the one to ask for advice on this particular subject, but I would just say that its so hard to come by trustworthy and communicative contractors/builders nowadays. I am not sure what courses to recommend to you, but I would say to build your presence in your local community and when you begin building homes just make sure you are available and go the extra mile for your clients. Just like anything else, I am certain that building relationships and receiving referrals for more builds starts there! Depending on your equity position in your duplex, you could look into taking out a HELOC or 2nd mortgage on that property to create more capital to go towards future building projects. From a realtors perspective and from someone who has dealt with builders who leave people "high and dry" in the building process, hopefully you can take something from this response. Good luck, man!