In German how do you say - i must get good marks and work harder?

There are several ways to say "I must get good marks and work harder" in German, depending on the level of formality and emphasis:

Formal:

* Ich muss gute Noten bekommen und fleißiger arbeiten. (This is a very straightforward and grammatically correct translation.)

Slightly less formal:

* Ich brauche gute Noten und muss härter arbeiten. (This uses "brauchen" which means "to need" making it sound a bit less strict.)

* Ich muss gute Noten erreichen und mehr arbeiten. (This uses "erreichen" - to achieve - and "mehr arbeiten" - to work more - which is a common way to express working harder.)

Informal:

* Ich muss gute Noten kriegen und mehr Gas geben! ("Kriegen" is a more informal way to say "get," and "Gas geben" is an idiom meaning "to step on the gas" or "to work harder.")

The best option depends on the context. The first option is generally safe for most situations.