Not sure what is the nature of your startup. But if you are building;
1. Web-based App
Decide on the programming language, the more popular languages are Java, PHP, Python, Rails, etc. Start by going through their Getting Started guide and make the 'Hello World' app. Once you are familiar with the programming language, seek out the related framework. Most of these web-based application already have a few established framework to help you speed up the development work.
2. iPhone App/Android App
Start going through the iOS SDK 4 docs or start by going through the Android SDK Dev Guide. I am still going through these myself. Can't provide more details here. Maybe the more experienced folks can chip in a bit.
These will get you started on the programming part. But working in a startup, you probably need to work as a team. So you will need to know a bit of the tools used in a team environment; version control tools like SVN or CVS, testing tools like junit, grinder, hudson, etc. The number of tools really depends on how your startup operate. The real way to get an idea what you will need is to Contribute To An Open Source Project; help to code, test, etc. Nothing beat experience gain from there.. Wish someone told me that when I was started out, then I won't have unlearn all my mistakes.