There's a wide variety of software out there for working with architectural visualization, with an even wider variety of design goals. What software you ultimately go with is going to depend heavily on what exactly you intend to use it for.
In general, architectural design software sort of falls on a spectrum from purely artistic software (IE: programs for creating realistic images or artwork based on design plans) to strict drafting software (IE: programs for making structural blueprints). I recommend that you look around at all the options before you decide to stick with any one software, but here are a few to get started:
Core Architectural Software:
Any one of these programs will more than likely have all the tools you need to do just about anything for architectural design and modeling, but finding one that matches your particular plans will help ensure that you have the most freedom and power in what matters most to you.