The first thing you'll need to do is determine exactly what kind of scale you'll be using for your project. This will set the stage for everything from how many bricks you need to how long your project is likely to take. You can make a pocket model, which is basically anything that's small enough to be carried in your pocket. At the other end of the spectrum is sculpture scale, which can be life-size or smaller, but is generally very large. In between is minifig scale, in which you plan the size of your design around the size of the minifigures that will use it. You might also choose to build a two-dimensional mosaic, using a base plate as a foundation.
There's no wrong way to build with Lego bricks, so different people's construction methods can vary greatly. But the first step will generally be research and planning. Here's what certified Lego professional and former Master Builder Nathan Sawaya had to say about planning a project:
Some people prefer to plan their creations out step by step, or to use computer software to help in their design process. Lego building programs include: