It may be a card from your hometown, or somewhere you went on vacation as a child. Each individual collector must decide what to collect. It is suggested that you accumulate a variety of cards at firs, then take on a couple of subjects. You may find that your favorite subject will change several times in the beginning. From a monetary standpoint, there are many subjects that have not yet caught the eye of the investors.
Many collectors focus on one artist or one publisher's cards.
Many cards are numbered, in sets or series, so you can have the fun of finding as many of them as you can. Remember that the quality of the postcard is important. There really is no limit to what you may find on a postcard if you look long enough. Long-time collectors will smile when a beginning collector announces what they collect. It always seems to turn out that way because as we go through life we find new interests on a daily basis.