To +infinite verb or verb-ing

By pyramiding the suggestions it is meant that the subject is given as many suggestions as possible which will require simple movements on his part, in order to acustom him to accepting the routine suggestions of the technic to be used for induction.

Hello, could anyone explain when I should use verb-ing, and when I should use infinite verb?

I learnt to memorise some phrases with infinite verbs behind "to", for examples:

what someone to do something

tell someone to do something

And I also was told to use verb-ing in the phrase of "look forward to doing something", because "to" is a preposition here.

Now I am confused by the usage here. How do I remember the usage of them?

Thanks!