Anything that exists is fair game for collectors. It seems everyone collects something. For some of us that is debt. The best known items for collecting are stamps and coins. Both are interesting and readily available to all, but the really great stuff is too expensive for us worker bees.
Collecting is at least as old as mankind and probably predates us. Among collectors there is a distinction between collecting the really old (antiques) and the not so old (collectibles). While it seems to me that anything one collects would be a “collectible”, those who collect antiques feel the need to be separated from the great unwashed. That’s OK by me though I do wash regularly.
Anything that attracts your interest is a good candidate for collecting. Porcelain figurines are a very popular area of interest. Most people get very specific with their figurines. They choose all angels, or dragons, or fairies, or whatever. Unlike stamps and coins, the best collectible figurines are not confined to the most expensive. Sometimes a majority of a collection may be porcelain, but enhanced by other things. I once knew a lady who collected owls. Many of the owls were porcelain, but many others were of other materials such as wood, fabric, different metals, and some pictures. It just had to be an owl to qualify.
I have over the years collected some things other than dust and debt such as glass insulators, chess sets, and for a while, rocks. Not gemstones mind you. Rocks. These were all brief interests. I m like an old hound. My attention is focused until the next rabbit runs.
Collectibles generally need to be either pretty, unusual, or have some historical significance. Pretty usually attracts the female collectors, historical to the males, and unusual grabs everyone, with overlaps in all categories. There of course, are other reasons for collecting and in some cases, no apparent reason at all. If asked, you may not be able to answer why you collect whatever it is that you collect. At any rate, the only criticism you have to listen to is your own.
Most people are collectors without being aware of it. If you should take note and realize that you have a lot of snuff boxes laying around that just seemed to accumulate, then you are in fact a snuff box collector whether you ever dipped snuff or not. Now you need to organize. Catalog each item and document every single fact you have, no matter how insignificant it seems. You may or may not be interested in the value of your collection, but over time, if you have really good documentation, your progeny may actually do something with your stuff other than dumping it.
Some people plan their collections in that they research the item before deciding to collect or not. These people are concerned with the collection as an investment. This is a risky business as collections of anything are only worth what someone is willing to pay. Beware of publications listing prices of collections. They can be disastrously misleading. Once you find yourself collecting something though, cataloging and researching each item will enhance not only the value of the collection but your pleasure as well.
Collections are not limited to tangible, inanimate objects. Many people collect fish, birds, dogs, cats, and other living things. Others collect intangibles such as travels, memories, sightings, and such. The intangibles would be hard pressed to be collections without documentation. While these collections have no monetary value, their worth cannot be measured by those doing the collecting.
There is a wealth of information on collecting on the internet. Much of it is general in nature, but with diligent searching, one can find information on collecting the specific of their choice. A starting point for your search is http://www.uniquegiftsandfigurines.com/