This information is outdated. Take it as you will.
There are a few problems with figuring out how to do things within the world of BlackBerry Development. These getting started posts will try to alleviate some of the problems caused by such things as:
  1. The getting started links after creating a BlackBerry Developer Zone login return 404 errors.
  2. Downloading and installing the correct JRE components often fails (with older JRE’s) and requires a login to an account for swdownloads.blackberry.com.
  3. Very difficult to figure out how to automatically compile for multiple platforms
After a few hours of searching I happened upon this link. Prior to this there appears to have been nowhere to find this information except through word of mouth. Since many BlackBerry.com websites often get lost in the void, a summary:
  1. Download and install the latest BlackBerry plug-in for Eclipse. Note that this should install a separate Eclipse: do not try to combine the installation that you’ve been working on for your Android application. Use two separate Eclipse instances for BB or Droid.
  2. Open it up after choosing a workspace and click the “Help” button on the top toolbar. Go to “Install new software” and add a new site named Blackberry Java Plug-in Update Site with location of http://www.blackberry.com/go/eclipseUpdate/3.6/java .
  3. Expand the “BlackBerry Java Plug-in Category and choose the SDK’s. Remember, the lowest version you can download (4.5.0.xx) is the lowest SDK you can use for your general library. So check the API reference to make sure everything that you need is there. This is due to the way that development for multiple OS’ work as is explained here  by Derek Konigsberg who created LogicMail.
  4. Optionally change the color scheme to what you are more comfortable with. If you do not like the default you can change the Eclipse background and colors by going to the same “Install new software” but using http://eclipse-color-theme.github.com/update/ as the site.
What that page does not tell you is that you will have a heck of a time downloading any SDK below OS7. After a few tries I found I had to do them individually, restarting eclipse after each install, or it would fail when asking for my credentials. I also found I had to create a new, fake, developer zone login. This login had to be confirmed (by checking email and clicking link) and had to have my information second checked and confirmed, which happens after your first login. After that, I was able to download the SDKs one by one.
My recommendation is to support only OS5 and up. This is due to the fact that majority of the devices that could be upgraded were. This is also due to the fact that it is a lot easier to do networking tasks (*todo, link to networking post) as there was an overhaul as of OS5.  It is also likely that the BlackBerry application player for QNX will better support 5 and up. However, if your device is a pearl flip for example, you are left with no choice but to support 4.6.1+.
The best information for a just starting BlackBerry java developer would be also from Derek at the site linked above. So that it is not lost, it is also here