For the basic screen Quicktime is sufficient. Open up Quicktime and go to File->New Screen Recording. A small control panel will open that allows you to control recording. The small dropdown arrow on the right gives access to various recording options, including sound. When you are ready hit the record button. QT will tell you to click to start right away recording the whole screen, or drag your mouse and select a part of the screen to record from. Then you get a 'start' button which you should click to start recording. If you have activated voice recording you can see your voice level in the control panel.
If you want to visualize your keystrokes on the screen (and don't want to spend money on separate software that does this in a fancy way) you can do the following: Go to System Preferences->Keyboard. Check 'Show keyboard and character viewers in menu bar'. This shows a flag on your menu bar corresponding to the current language. Click the flag and check 'Show keyboard viewer'. This will show a schematic keyboard on your screen which lights up with the keys you press. There is no persistence, so it is not as apparent as specialized software made for screen casts.
Additionally, if you have a cluttered desktop, from a hint here, you can hide all the icons as follows: In a terminal type
defaults write com.apple.finder CreateDesktop -bool falseAnd then restart Finder. To unhide the icons type
defaults write com.apple.finder CreateDesktop -bool trueAnd then restart Finder