The migration from Windows to Ubuntu is not very difficult, but also not very easy. It's like moving house, everything feels a little out of place for a while. For a smooth migration I suggest the following:
1. Install Ubuntu first as dual boot beside your Windows installation.
This gives you the opportunity to get used to Ubuntu and to step by step migrate the different applications. Some applications will be exactly the same as in Windows (Firefox, Adobe Acrobat, Skype, etc.) and some will be very similar (Office applications). For others you may find applications who can perform the same tasks, but who are quite different to use. In very rare cases, it might be impossible for you to find the application that performs the functions exactly as in Windows. In this case you can use WineHQ (see Use WineHQ to run Windows Applications)
2. Start with Ubuntu 12.04 LTS
Ubuntu 12.04 LTS is not the newest version, but it offers you long term support till April 2017 (LTS = Long Term Support). This version of Ubuntu is not much different than Windows, but it has a user interface that apparently needs getting used to. If you are a beginner, I recommend you to use the LTS versions only.