Focus on the end-goal instead of the road there
You might easily find yourself wanting to complain about an essay that you have to write for next Tuesday, or the list of vocab that you have to learn for your test tomorrow, but you might find it a lot easier to face these tasks if you force yourself to really think about why you're doing them.
In the case of the essay, you're doing it so that you can practise, so that your teacher can correct it and so you can learn from your mistakes, so you can get better, so that finally, you can get a great mark, feel a sense of achievement and if you're in 3rd or 6th year, get the results (or college course) you want!
Focusing on the future and focusing on the reasons why you're doing something that you mightn't particularly like can make it a lot easier to carry out. Also, remember, everything comes to an end eventually, and so will all the study and homework, etc. It won't last forever, so concentrate on the future, when you'll be finished, and let that be your overpowering, positive thought, instead of a potentially negative one that could end up making the task a lot harder for you.