The first line in a work of fiction can often be the most difficult to write. As an author of a fictional piece, you are transporting your reader somewhere. It may be just up the street to a seedy bar, or across the galaxy to a planet in peril. Regardless, you have one chance to make a first impression--with a good opening line.
One way to begin is to open your story in the middle of action. For example, if you are writing a buddy-cop piece, put your protagonists in the middle of a gunfight, not sitting at the station discussing a case over coffee. By dropping the reader into the middle of the action, you're raising questions in their mind. They will want to know how your characters got into this situation--and how they're going to get out of it.
This kind of intrigue can also be accomplished through putting the reader inside the head of one of your characters. If you open with a thought, this will help the reader to orient themselves inside the story through the perception of the speaker. They will want to know who they're listening to, and why they're thinking what they are. It probably goes without saying that this should be an interesting thought, not musings on the subject of lunch or which tie to wear today.
Also, don't limit yourself to just the hero; opening from the perspective of the villain, the sidekick, or the feisty love interest in peril can provide depth, as well as an unexpected twist for the reader. Opening with a description of the environment is usually a failsafe for the fiction writer, because it's what we as authors see most clearly in our own minds. However, this can be dangerous as it can lead to a long descriptive paragraph sitting there dragging down your piece from the beginning.
If you just can't let go of the idea of opening with a descriptive passage, ensure that it is either interesting enough to hook your reader or that it is limited to a few lines followed by action. A good opening line is one which encourages the reader to proceed on to the second, and then the third. It has to sparkle. If none of these tactics are working to produce that perfect-feeling line, just open with whatever you are thinking and work backward later. Often I will write a pretty bland set of opening sentences, only to find the really good opening line hiding a few paragraphs down.
Sometimes it's all about getting the creative juices flowing and being willing to reorder your thoughts if it is more engaging to the reader. Lastly, avoid the cliche. As authors, we write from what we know, and it can be easy to fall into traps and use words and phrases we have heard over and over. Be sure to pick creative adjectives and dust off those old unused turns of phrase your grandmother taught you. These tactics will help you produce something memorable, magical, and often elusive--an opening line worth following up on.