As the creator of a new app or website, you are intimately familiar with its purpose and you believe deeply in the value it offers. But unlike you, when a new user arrives at your app or site for the first time, he will be neither familiar with it nor confident of its value and trustworthiness. It's your program's responsibility to make the new user comfortable, knowledgeable, and confident about its purpose in the first few moments they are together.
It's very much like when two strangers meet on the street. Even if both parties have good intentions, it is imperative that these intentions be made abundantly clear and unequivocal before any significant interaction can take place. Eye contact, hand shaking, and smiling are the cues used in real life, and the web designer must provide equivalent cues on the screen. When someone sees a website for the first time, several important questions come instantly to mind. The first, and most important, question is, "What the hell is this thing?" The program should answer that question using no more than a phrase. Sometimes the product name is sufficient, but typically a subtitle or image does the work, but if your program doesn't make this clear in a glance, you have some significant design work to do. It is surprising to me how many websites fail to answer this most fundamental question.
The next questions that will occur to the user are, "What does it do?" and "Why would I want that?" At this point, some text can be used to provide answers. Lighten up the text with a diagram, drawing, or image. You don't necessarily want to burden repeat visitors with this stuff, but your software can easily tell the difference between a first time user and a veteran.
Once the user knows what the website is, what it does, and why that would be a good thing, he or she can understand the advantage of having such a product. So now she will be more willing to pay closer attention to more granular questions, such as, "Wouldn't it be easier to just use something else I already know?" Here's where the site can offer comparisons to similar products and itemize its qualities. Any potential user will be weighing the benefits of your program against the burden of learning something new. Make your program easier to learn and use, then prove it.
At this point, the new user is likely asking himself, "Is this going to cost me money?" and, "Do I have to give it a credit card?" Be up front about this now. Don't be coy by only telling the user that money is involved after they have pushed the "Yes, I Accept" button. Tell them now and disclose the full amount. Honesty now will result in more trust, which means more click-throughs and more happy users.
Everybody knows that money isn't the only thing a website can cost. Experienced users will ask, "Will it steal my private data?" and "How long is this going to take?" Once again, take the time to honestly and completely answer these questions. Give the user the option to use the program without surrendering their private information. If they like your program and use it regularly, you can ask them for it again in a month and their answer might well be different.
Good user experience design will keep any user's time overhead down to a minimum, so you should be able to give them good news in the beginning. Saying something like, "This takes the average user 43 seconds." Big picture information like this goes a long way towards assuaging the user's worries.
After your website and the new user have performed this little pas-de-deux of introduction, the human at the other end will be far more likely to end up being a satisfied, long-term user of your program.
Image source: Nightdeposits.