Some interactive fiction games (like "The Closet" and "Pick Your Poison") have a choose-your-own-adventure structure. Others, such as "The Study" and "Hide and Seek", are parser-based: you type commands at the bottom of the text in order to move around and interact with objects and characters. These types of games can be more challenging because you aren't necessarily told your options or how to move the story forward--but I think exploring the world is part of the fun!
If you are new to playing IF, you might find these commands useful:
Go east, west, north, or south.
Examine [object]. For example, you might examine the computer.
Put [object] on [object]
Sit in/on [object]
Of course, there might be other commands specific to the game you are playing. If you've been frustrated trying to play my games, keep an eye out for walkthroughs of "The Study" and "Hide and Seek". I should have them up in the next couple of weeks.
If you're interested in a more in-depth tutorial, I recommend The Dreamhold. It was the first IF game I ever played, and I've been hooked since!