There are two main inroads to the foreign service being the State Department and US Agency for International Development (USAID). The recruitment process has some similarities but also many differences. Also, the type of candidate background each one is looking for is also pretty different. I'll be working for USAID.
I went through the process for both so in upcoming posts, I'll try to relate my experiences for both of them.
For those who are interested Wikipedia has a pretty fleshed out article on the foreign service going into the history of its creation and much information about it.
Also, for those who might want to really understand the foreign service and the job, I highly recommend the book "Career Diplomacy". Especially if you might consider a career as an foreign service officer (FSO), then this book will be a great resource.