1. We decided going to the cinema.
2. My cousin is going to Canada, so she must to get a passport.
3. He promised to help me find a holiday job.
4. She needs buy a new schoolbag.

1. We decided to go to the cinema.( после глагола decide употребляется инфинитив с частицей to )

2. My cousin is going to Canada, so she must get a passport.( после модального глагола must глагол употребляется без частицы to)

3. He promised to help me to find a holiday job. Или

He promised to help me find a holiday job. ( help somebody do ( to do) something

4. She needs to buy a new schoolbag.( need to do something - что-то нужно/ необходимо сделать).

Объяснение:

1) We decided to go to the cinema.

2) My cousin is going to Canada, so she must to get a passport. (тут всё верно)

3) He promised to help me to find a holiday job.

4) She needs to buy a new schoolbag.