New with Blender…
Some how I think I’ve managed to change/screw the preferences so that when I press Ctr-Xkey (or menu File->New->Ok) it won’t bring me back to the “start up” scene (the one with the rectangle…by default).
Also, when I hit Tab I can’t go to Edit Mode…I’m stuck on Object Mode.
Maybe this are simple issues for experienced users, so thanks in advance 
Halcyon
Intel Mac, OSX10.4.7
Ctrl-U saves a new default scene in a file called b.blend. If you can find and delete this, Blender will build a new default>default file on re-start. Otherwise, just set Blender up any way you’d like it to be on every new file and press Ctrl-U to save this as your new default.
Take note that just about everything will be saved exactly as is - including cameras, lights, objects, file paths and names, colours, window arrangements and more.
Inability to enter EditMode is usually because you have either nothing active/selected or you have something uneditable selected (lamp, cameras etc…). If you have a mesh selected but also have a lamp, camera or empty selected, you may not be able to enter Edit Mode.