You're Certified. Now What?

You've passed your certification. It's official, you're a USSF referee. Now what?
After certification, there are a couple of things you'll need to do.

1. Purchase uniforms, whistles, cards etc.
2. Contact assignors

Purchasing Equipment
There are many places to purchase uniforms as well as the accessories necessary to referee a soccer match. The official USSF provider of uniforms is Official Sports, Inc. There are many, many other sites. One in particular that I recommend, is Score Sports. Their uniforms are very high quality and a little less expensive. See my list of links for other equipment providers.
I recommend the following items for someone just starting out as a soccer referee:
Uniforms:

• 1 pair of referee shorts
• 1 pair of referee socks
• A good pair of black or black and white turf shoes
• 2 long sleeve jerseys, one gold and one other color (assuming you'll encounter cool weather)
• 2 short sleeve jerseys, one gold and one other color

Accessories:

• Watch - Any watch with a chronograph function will do
  
• Flipping coin - a large, heavy coin like a half dollar will do
• Red and Yellow Cards, data notebook
• Flags
  
• Pens and Pencils
• A bag to carry your stuff
• 2 whistles

Getting Assigned
I thought getting assignments would be easy after getting certified. I was wrong. I suppose it probably varies by state, but I found the assignment process to be confusing. Your mileage will vary. First, you have to get in contact with a few assignors. The easiest way to do that is contact your state committee to get a list of local assignors. An alternative is you can contact the various leagues in your area and get the contact information for the league's assignor.
Once you get a list of a few assignors, call or send them email. Mention that you are a newly certified referee and ask to be assigned. Be persistent if you don't get an answer. My advice is to take whatever assignments you get and do a great job with them. That's how you are going to get more and better assignments.
You shouldn't expect to get assigned to high-level matches right off. It is very likely you'll get assigned as an Assistant Referee, hopefully paired with an experienced center official. Your first experience as a center referee will most likely be with very young players and less skilled divisions.