In general, you are going to want to stay away from partnership relationships which most would argue is the worst entity choice one can make because not only are you liable for your actions, you are also liable for the actions of your partners. Start by getting a basic book on entity selection at the library or bookstore, assuming you have time so as to get up to speed on the basic differences. The contact a corporate or business attorney that specializes in the area. I would not recommend doing this on your own even though many books would tell you it is easy. There are nuances and the choice you make may depend on your business. LLC's have become very popular although relatively a new concept in our legal system if you consider the amount of case precedent versus that of the statutory corporations. Also, be aware there is a new trend by some advisors to move people back into C corporations, although most cpa's and attorneys would tell you to steer away from double taxation. I'm not sold on this new train of thought but it is out there and you need to really look at your business objectives long term to get a better feel as to what you are trying to accomplish. At the end of the day, I'm a big fan of LLC's but definetely speak to local counsel for the best guidance. Hope this helps.
In WA, the paperwork to form an LLC or a corporation is quite simple. The forms are free at: http://www.secstate.wa.gov/corps/registration_forms.aspx .
In fact, most the registration done with the state can be done online.
However, you likely want to discuss with an attorney the pros and cons the various business entities.
Since you mentioned partnership, this likely means that you are going into business with at least one other person. Have you and the other person discuss all the things that can come up when working together?
One of my main practice areas is representing small businesses, from corporate formation to contracts to windup, and everything in between. Feel free to contact me (425.275.9458) to schedule a free 30 minute consultation regarding your corporate formation questions.
Although you can fill out the forms yourself, many of my small business clients come to me because of a defect in their DIY legal forms. I strongly encourage you to hire an attorney for this process, at the very least to review the documents you complete before filing them with the state.
Also, the best corporate structure for your business will largely depend on the specifics of your situation - what kind of business you are starting, your personal situation, whether you will have investors/partners, etc. Be sure any advice you receive is tailored based on that information, rather than just general in nature.