Hi,
As mentioned the Arduino site is where you begin though there is a world of difference between hobby projects and the professional world.
Look around any you will find many suitable Arduino tutorials .
Here just one
https://www.element14.com/community/videos/7910
Start to build up your skills on the Adruino by all means and keep detailed records/ portfolio of projects you have actually written so you can show prospective employers etc.
Most chip makers have their own micros out there these days, Microchip, Texas, Cypress, Freescale, ST, to mention but a few, so would think anyone looking to move into software / hardware design would need experience of at least a few others as well.
Going 'freelance' is a much tougher challenge, you really need both the necessary skill sets and ideally the experience of have worked within the industry.
Web based adverts for freelance/ work at home Arduino programmers just sounds like an advert for suckers, do all the programming but try and get the money !