If you come from a completely different background and just starting to venture into starting your own tech company, which skill would you learn first?
Programming, web design, or sales?
Which skill are you most passionate about? Start there!
All of these skills take a good deal of time to become proficient. Start with the one that interests you the most since you'll be investing the majority of your time doing it.
While you're working on a single skill, read up on the fundamentals of the other skills. Figure out the major players in that field and follow them. Work on building relationships with people who have the other skills you need (hint - they're likely doing the same). When you're ready to assemble a team you'll be perfectly positioned to tap your knowledge and your network.
Someone once told me "Sales solves all challenges". While that isn't necessarily 100% accurate, is a great message. Why? Because if you're selling successfully you accomplish these things (assuming you have a product/service):
A great CD to start with is Question Based Selling by Thomas Freese. Another great book is High Trust Selling by Todd Duncan. Then invest the money and hire yourself a sales coach.
None of the above.
Instead, learn how to convince people to work together with you and then do what you're best at and/or makes you happiest.
It really depends on what kind of team you have / are planning to assemble. Let me say first that it can be hard to learn programming. You might get an idea of what programming is in a few weeks and even write some code in a month or so but when it comes to facing real life challenges, even programmers with years of experience sometimes stumble.
Personally I've always looked at design as a thing of talent. You either have it or you don't and thus you are the one to know if you can do it or not.
Sales is another story, an entrepreneur "must" be skilled in sales to a some point. Because he/she has to know how a product will sell in order to instruct coders and designers with the income value at mind. A lot of brilliant projects failed because they had no income value and lesser products became a hit because their sales startegies were very well modeled.
My personal recommendation would be to learn a bit of both. Learn how to design user experiences (not the actual graphics) and use those creations to augment a well implemented sales startegy.