I'd have to disagree on his assessment of editors. You don't have to spend a fortune to get started. Ulead Video Studio on the windows platform for example provides an extensive set of tools in its editor with support for multiple HD formats in addition to other standard formats and it's less than $100.
It's considered a consumer product however the capabilities go beyond what you expect for its price tag and more than anyone starting out would need especially if they have a small budget. Most of the major features are similar to what you would expect from a high end editor just a few years ago. The more advanced editors like Vegas and Premiere have a much higher learning curve but do provide many more capabilities. A lot of these features will never be used in any basic production.
If you have the money for a high end editor and more importantly the time to learn how to use it go for it. Otherwise I'd suggest sticking with more user friendly applications at least to get your feet wet.
Sorry for the double post but I'm past the edit time. I just wanted to add there is a plethora of free video applications available on the windows platform if you want to go that route. Most of these are simple tools made to do one task and usually perform them better than anything else you'll find including high end video editors. There's also more advanced tools like Virtualdub and Avisynth. Most of these are going to be designed to work with standard video formats but in time they will catch up or others will emerge for HD.
There's no free replacement for a good editor that I'm aware of but you can certainly supplement your "arsenal" with the free tools available.