I need to crack on
You might say this when you have been chatting to your friend for a while and then you want to say, “I need to start doing my work”. It means, “I need to get on and do some work” i.e. because you have just been doing something that was not work e.g. relaxing, chatting, etc. Another way of saying this is, OK, I had better get on with my work.
I need to crack on if I’m going to finish this essay by 5pm tonight.
Don’t you think we had better crack on with the cleaning if we want to have time to relax this afternoon?
I want to crack on with my essays tonight.
So often we say crack on + with + something but also you can simply say, crack on, without anything after it. This is colloquial, spoken English.