The Vertical or Straight key is indeed the simpler (mechanically speaking) of all CW devices.
This key must be literally "played" like an instrument.
 In this video, former RT Officer Oscar Portoghese I7OHP, shows how to correctly manipulate a straight key.
In transmiting CW, timing and spacing is crucial. The basic unit of measure of CW is the dot, all other elements are based on it.

• Dot: 1 unit
• Space between Dot and Dashes: 1 unit
  
• Dash: 3 unit
• Space betweem letters in a word: 3 unit
  
• Space between words: 7 unit

Do not force yourself to learn these rules by heart: assimilate them while transmitting with the straight key, hear the sound as transmitted by a PC and try to play it back exactly as you heard it.

The correct use of a Straight key is a fundamental prerequisite for learning CW, relatively few people are able to jump directly to the bug or a paddle.
Always keep in mind CW motto : "Accuracy trascends speed" !