Learning English language requires an hour or more out of one's daily schedule if someone has just started learning it. I tell you a bit of my story that when I started doing my masters, I was quite bad at English, and I literally could not even write or speak a few lines of it but as soon as I realized the importance of it, I had set my goals that I would learn it gradually no matter what. So I would always give time once in a week until I became an aspirant of CSS Exam, and now I daily spend some time learning it to maintain the rhythm, and I recommend you to give it a proper time but on daily basis so that you may never lose the interest.

For competitive exams one must have good writing skills in English language in order to easily get through this exam, and to improve your writing skills you will have to READ and WRITE extensively on daily basis along with improving your GRAMMAR and VOCABULARY. These four things along with a few other things which I'm going to mention, will InshaAllah make you good at English language.

In order to improve your grammar, you will have to make yourself familiar with and good at the following things.
1) Parts of Speech
2) Tenses
3) Active Passive Voice
4) Direct Indirect Speech
5) So many grammatical rules (Don't worry you will learn them gradually).

As far as vocabulary is concerned whenever you find an unfamiliar word, write it down on note pad and then consult dictionary. Knowing the meaning of the word is not enough, you must also see how the word is used in the sentence and also know whether which part of the speech it belongs to, means whether it belongs to noun, verb, adjective, adverb etc. Daily learn new words but to retain them in you memory your memory, revision is compulsory. And you have asked about how to correctly use a word in a sentence as you find difficult. So brother it problem will gradually reduce when you you will extensively read and write. Learning Idioms and Phrasal Verbs, Confused words should also be part of your schedule.

Lastly I want to say something to you that this whole process of learning English requires daily practice and patience, so don't ignore either one

Books : Go for the books which FPSC recommends.(Would be better)