That's a tricky one. I see several possible ways to go here:
1. Use an established taxonomy of skills (say from the Bureau of the Census). Tweak it as you go along.
2. Start running the database allowing any folksonomy tag to be input. After a while, collect statistics on the most used tags, either keep them as a folksonomy with, say, a suggested list, or try to build a taxonomy using the list.
3. Analyze your domain in advance and build a taxonomy.
Please see Clay Shirky's claim that you do not need any taxonomy in the links.