Great advice in the two articles linked above by
@Samer.
I would suggest that, at least at first,
you stick to areas that you know well - e.g. if you are into travel look at travel domains, if your full time job is in selling technology maybe consider domain names in that niche, etc. There are often small nuances that make all the difference in a good or not name that someone who really knows an area will see instantly.
Secondly,
make sure the domain name is the correct and best wording. Yes, wrong order, creatively spelled, etc. names sell, sometimes for a lot, but in general businesses want the respect of a correct name.
Third, but really it should be first,
before you buy a name make a list of who,in general terms,
might use it.
Related to that,
ask yourself whether you would start a business on that domain name. If not, move on.
Make sure you are getting good value on a domain name acquisition. A name might be great at $15 but a poor investment at $200 if the likely selling price is only going to be $600 and it may take many years, if ever, to sell even at that. Research every possible related sale at NameBio. Yes, it takes time, but you will save that time later when trying to sell your names.
Not everyone will agree, and it will cause you to lose some opportunities, but i think it is always good to
sleep on the idea of the domain name prior to acquiring it. Do your research, almost make up your mind, but wait until the next day. If you feel less sure about it, move on.
There are many domain fish in the sea.
Don't feel you need any particular one name to be successful. If you do get in that mindset you will pay too much at auctions or hand-reg too many impulse purchases.
Best wishes in domain investing.
Bob