The easiest way to build your vocabulary is through reading. Hearing (e.g. while watching a movie etc.) and speaking (imitating and recalling the words that you heard) helps a lot to learn faster. Start from learning the basic words that we use in daily life/routine. Use those words in your conversation, this way you won't forget them. You may combine English words with your native language. For example, I'm learning Mandarin and if I want to say 'Please pass me that book', I can say it like ''请(Qing=Please) pass me that book'. Or I can say, 'I like 苹果(Píngguo=apples)'. I think practicing the words we learn like this, helps us memorize them permanently.

Though everyone is different, so it might work for some people and not for others.

Yusef, how is it going ?

I wont write as much as the previous people.
I would tell you :

1. Repetition

Read it, write it, re-read it, re-write it,text it, use it.

2. Context

If you write the word in a sentence, your brain will store it faster than if you just read a flashcard. 
Flashcard is a trendy tool, but very limited actually. 
Write sentences ;) 

If you need more tips, feel free to write to me. 

Enjoy

There are two ways of learning words: or by repetition or by inventing once any helpful trick.

Although every student has other preferences, I would say that  in the first case the best way is by a Spaced Repetition System, already described in other comments. Always try to add to the word a sample sentence. Memrise.com might be a great help if you don't want to create them on your own. For example B1 level in German is about 4000 Word and it will be easier to handle them digitally than with paper flash cards (in my opinion)

For the second, try to invent anything that let you remember more easily. For example if you mix up migi and hidari (one is left in Japanese, the other right) then remember the sentence "I would like to have the rights of Migi Mouse" and you will never forget that migi is right. Try to find anything similar in other languages, things that come to your mind when you hear the word. Imagine Shakespeare shaking a bear (or a beer). No help is too stupid. It is the opposite. The more stupid the rule, the easier it is to memorize.

Adding on to what Kyrsten said, I love SRS flashcards! Although I actually like using apps, as then I can study anywhere with my phone. I personally like to use Brainscape. When I find a new word, I add it to my flashcard deck and then add multiple example sentences. Example sentences are important because you want to make sure to learn words in context! 

Other than flashcards, just try to absorb as much native material as possible. TV shows, books, blogs, news articles; Whatever you like! You may not understand the material completely, but you'll see certain vocabulary used over and over, which is the best way to retain it in my opinion.