It's tricky... If you're interested in getting into domaining (domain investing) and just starting out, I would say the most important thing to do is WAIT. What I mean is, don't spend any money until you've thoroughly researched the industry, learn what sells, what doesn't, how to acquire good domains, look for niches, etc. It will save you money in the long run. I didn't do this when I first started out and spent a bunch on crap domains that didn't sell... I've learned and now have sharpened my skills but it takes time and dedication to learn the tricks of the industry.
Thanks for the comment @senstless!