Focus on that one domain
Many in the SEO world will say that you should buy as many domain names as you can afford, point them all to your main website and optimize all of them. This is a great strategy if you can spare the time and cost of optimizing each one of those URLs. The fact is you can always do more to optimize that one main URL. If you start trying to spread your efforts out over dozens of domains you run the risk of diluting your results.
For now you should stick to optimizing that one main URL and spend the rest of your time keeping up with the latest SEO strategies.