The Japanese verb "To do - Suru" is the most important verb to learn.
The plain form is - Suru する
The polite form is Shimasu します
Saying Suru or Shimasu alone is like saying speaking in a future tense, it means "I will do it".
Many Japanese nouns take the + Sure form which makes it easy for learners of Japanese.
For example Diet wo shimasu ダイエットをします。Means I am going to go on / start a diet.
The plain tenses are as follows.
Shita した - to have done / did
Shitai したい - to want to
Shitakunai したくない - to not want to do
Shiteiru している - to be doing
The polite tenses are
Shimashita しました - to have done / did
Shiteimasu しています - to be doing.
Shitai an Shitakunai + desu will make the polite form.