The most important thing to know about buying art, either as a one off or as the start of a private collection, is that there's no list of things that you need to know before you jump in. If you see a piece of art that you absolutely love and you want to buy it, then buy it. You don't need to have credentials or a degree in Art History to start a collection, you just need to have a desire to collect and the means to pay for it.
The more you expose yourself to art and the 'art world', the more you will be able to 'evaluate' what is fundamentally good art, what suits your taste and how to build & develop your collection.
If you are collecting for investment purposes then you will need to research as much as possible and, more importantly perhaps, build relationships with good quality, experienced dealers and galleries who can advise you further.
If, however, you are collecting for your own enjoyment in the hope that your collection will increase in value over time, make sure that you like what you buy. It is you that will live with the art for many years so it has to be something that you are confident you will enjoy.
Click here for more help & advice.