I've seen people use those programs that model out your own LEGO design so you and others can see how it would look. Can I get a list of a few? Free and safe are preferable. Thanks!
The most commonly used one is Lego Digital Designer (often called LDD). This has the advantage of being free and quite easy to use, including automatically snapping connections together. However, it is only available for Windows and Mac. Also, LEGO has discontinued support for it.
A recent arrival is Stud.io. It's in open beta but seems quite user-friendly. It also has the advantage that it integrates with BrickLink. It will tell you how much a certain part (or your whole model) will typically cost on BrickLink, and if a certain part is unavailable in a certain color.
One caveat is that none of the above solutions has all available parts in it. Most common parts are available in all three, but some of the less common or older parts are not. Some parts -- especially the very new, or very specialized parts -- are not available in any of them.
Another program that you can use that runs in the browser and doesn't require any downloading is Build With Chrome. It allows you to use a bunch of bricks to create a model. You can browse other people's models and create your own. You can save your models by signing into it if you have a Google Account.
UPDATE: 8/8/19 Unfortunately the website was removed as of March 2019