There are several ways to prepare for vocabulary tests, some of which will give the best long term results and some of which may help in the short term.
If the test is over a small group of vocabulary words, a tried and true technique for studying them is using flash cards. Take a set of index cards and write the vocabulary word on the front and the definition on the back. Set aside four 15-minute study sessions during the day, as this is more effective than a single long session.
In the first session, look at the fronts of the cards, read the vocabulary word, and try to remember the definition. Turn over the card to see if your memory is accurate, and if not, read the definition aloud, as you remember more of what you read aloud than what you read silently. Do this for every card.
In the second session, look at the definitions on the backs of the cards and try to remember the word being defined. In the third session, look at the fronts of the cards again and in the fourth session look at the backs of the cards.
Over the long term, the best way to develop your vocabulary is reading books written for adults rather than children which use a wide vocabulary and looking up words you don't understand in a dictionary. If you spend an hour or so every day reading, you will develop such a good ordinary vocabulary that you won't even need to study for vocabulary tests because all the words will already be familiar to you.