According to the US Patent and Trademark Office's handbook:
A trademark is generally a word, phrase, symbol, or design, or a combination thereof, that identifies and distinguishes the source of the goods of one party from those of others (1).
In order to register the trademark you will have to show:
(1) that the trademark will not be confused with other already registered marks--espeically those associated with similar services.
(2) secondly you will have to show that the mark is unique enough to be protectable. If a mark merely describes the product, it is not registerable. The manual gives the example: "creamy" as a non-registerable trademark for yogurt (3). Instead names should be suggestive.
Those are the two main criterion.
Also, however, the trademark can be denied if it is:
a surname; geographically descriptive of the origin of the goods/ services; disparaging or offensive; a foreign term that translates to a descriptive or generic term; an individual’s name or likeness; the title of a single book and/or movie; and matter that is used in a purely ornamental manner (4).