Just take it bird by bird.
As an SAT and GRE tutor, I routinely encounter students who are overwhelmed by the amount of words they have learn - whether because English is not their native language, or because they looked at an SAT vocab list and wound up cowering under the bed. Here's the truth: there are always more words to learn. You can never learn them all.
But you can make an effort to learn words that will help on whatever test you are taking - SSAT, SAT, GRE, TOEFL, or any other - and greater command of vocabulary will not only improve your score but also increase your confidence taking the test.
When students come to me for GRE or SAT vocab help, they ask me, "How do I get over the fear of the endless vocabulary list?" My response is, by starting somewhere, and chipping away slowly at that mountain of words.
One of my favorite books on writing is Anne Lamott's Bird by Bird, and the eponymous story is one that has always stuck with me:
"...my older brother, who was ten years old at the time, was trying to get a report on birds written that he'd had three months to write, which was due the next day. We were out at our family cabin in Bolinas, and he was at the kitchen table close to tears, surrounded by binder paper and pencils and unopened books on birds, immobilized by the hugeness of the task ahead. Then my father sat down beside him, put his arm around my brother's shoulder, and said, "Bird by bird, buddy. Just take it bird by bird." "
This story has stuck with me because it's a feeling most of us have experienced - whether facing a large writing project, studying for all of the SAT, or even approaching just the vocabulary portion. There's so much information that it can seem impossible to start. But it's important to start somewhere. (for example, if you don't know eponymous, go look it up!)
Cambridge Coaching GRE and SAT tutors usually advise their students to learn 50 - 75 new words a week. You can only do that with careful planning. Studies have shown that most people need to have exposure to a word 12-18 times before it is stored in long-term memory.
What does this mean for you?
Do a little every day. If you do the math (quick, can you beat me to it?) 50 words/week = 10 words each day Monday through Friday. Easy! 75 words/week = 11 words Monday-Friday and 10 words Sat-Sun. Sitting down and writing out the definitions for 75 words in a row would be torture for most of us - break it up.
What do you write when you write those words? Your key word, the primary definition and secondary definitions. If you want the best chance of remembering the word, you should also write a sentence that uses the word in context. Or draw a picture that shows the meaning (this can be a creative break: how can you show the meaning of "perspicacious" with a stick figure? These abstract drawings sometimes comes out so strange that they help you remember the word). Yes, you can buy any of 1,000,000 vocabulary books on the market, but that will not replace the process of your making a flashcard, which is helping encode the information and helping you learn.
Do a little every day means you only have to add a few at a time - but look at the second part: every day. You may only add 10 words a day, but you also need to be reviewing your other words. That's where the repetition comes in.
How to review? Everyone is different. Some people prefer smartphone apps - there are endless flashcard apps, or quizzes and games with websites like Quizlet. I still prefer pen-and-paper notecards, or a vocabulary list with the words on the left and definition on the right that I can fold over. Maybe you can enlist your parents to utilize your vocabulary words at breakfast, like Jeremy's parents -- or maybe you'd rather not. The more you use your new words every day, however, the more they will stick.
Where to get these words? Cambridge Coaching SAT and GRE tutors have ample advice depending on what test you are preparing for, and those lists have been built from years of experience. The words you are most likely to remember, however, are the ones that you come across in your reading in everyday life. Make it a habit to notice new vocabulary as you read, and write down the word to look up later. You already have an authentic context someone else has used it in.
Whenever you are studying vocabulary, you may be preparing for a test, but you are also enriching all of your future writing, and creating good habits that will help you as you go on to study other languages.