You asked some pretty good questions and the the biggest one is number four. The answer is "a lot".
I'll start with each of your questions if I can.
1. How do you go about obtaining a Part 135 certificate?
First you might start by going to
www.part135.info and viewing the video on thier site. The video is the same one that you are required to view at the FSDO. You are required to view before you can even apply for a 135. It will explain the 5 steps to obtaining a part 135 certificate.
2. How do you create your Ops Specs?
The Ops Specs are created by the FSDO for you during the certifications process.
3. How do you determine aircraft operating costs?
This is the toughest question. By far the biggest cost will be insurance. Especially for someone with so few years.
4. What else am I forgetting?
You are forgetting a whole bunch. This is not an easy project. You might want to talk to some one who does this all the time. When I started my 135 I used Aviation Compuer Systems. They were very helpful and they did almost all the paperwork for me. They started me with a single pilot operation and about 300 pages of stuff to submit to the FSDO. They made all the revisions for me and mostly made it a no-brainer for me. You can reach them at 888-PART135. I think the website is part135.com.
I hope this helps