From BlenderWiki

Menus

Blender contains many menus, each of which is accessible from either the window headers or directly at the mouse’s location using hotkeys.

For example, you can access the Toolbox in the 3D window, either by pressing Space on the keyboard, or by holding down the LMB or RMB buttons for a few seconds on the mouse. The Toolbox will then pop-up at the mouse position. The top most menu of the Toolbox can be seen in the (Toolbox’s top level) image.

Some menus are context sensitive in that they are only available under certain situations. For example, the Boolean Tools menu is only available in Object mode using the W hotkey. The same hotkey (W) in Edit mode brings up the Specials menu.

While you are using Blender, be aware of what mode and types of object are selected. This helps in knowing what hotkeys work at what times.

Because Blender doesn’t use the standard OS menu system, if you are using a Mac, you likely have a redundant menubar at the top. To remove this see this post on Macworld, but beware that it is somewhat complex.