When something 'expands' it means to grow or gain in size. English is one of the most expressive and emotive languages in the world so some students can find the amount of words daunting.
I would like to add more words to my vocabulary. Do you have any ideas?
Do you know how I can expand my vocabulary?
What should I do to make my vocabulary better?
Any language is always strengthened by the vocabulary you know. When meeting someone of the language you are learning and you would like their help, all you have to do is ask them "Do you know how I can expand my vocabulary?"and they will offer you solutions and ways to do that. One of the ways you could do that is by listening to English music and watching English shows!
If you want to do something in the future, you can explain it by using the form: 'I am hoping to + VERB.
Eg. 'I'm hoping to study at Oxford University when I leave college.'
将来何かをしたいなら、'I am hoping to + 動詞' を使って表すことができます。
例：
'I'm hoping to study at Oxford University when I leave college.'
（カレッジを卒業したら、オックスフォード大学で学びたいと思っています）
I would like to learn more English words and what they mean.
Will you help me learn more English words to expand my vocabulary?
I need to know what that word means so I can know more words.
You want to expand your vocabulary or grow the amount of words you learn but some people can be put off by you saying "vocabulary" so just say "words". It comes across as more friendly.
I would like to learn more English words and what they mean.
Will you help me learn more English words to expand my vocabulary?
I need to know what that word means so I can know more words.
I need to remember that word so I can use it in the future.
ボキャブラリーを増やしたいということですが、"vocabulary"という単語を使うと興味をなくす人もいると思います。ですから、"words"を使いましょう。よりフレンドリーな印象になります。
I would like to learn more English words and what they mean.（もっと英単語とその意味を覚えたいです）
Will you help me learn more English words to expand my vocabulary?（英語のボキャブラリーを増やすのを手伝ってもらえますか）
I need to know what that word means so I can know more words.（ボキャブラリーが増えるようにその単語の意味を知りたいです）
I need to remember that word so I can use it in the future.（これから使えるようにその単語を覚えておかないと）
The two sentences you see provided above are excellent ways to express to your listener that you want to expand your vocabulary. In the first sentence you will see the word grow and in the second sentence you will see the term add to. This word and this term mean to expand. They would make great additions to your vocabulary.
I want to build my vocabulary' and 'I want to grow my vocabulary' and 'I want a larger vocabulary' have the same meaning as 'I want to expand my vocabulary.' To learn more words and to be able to use those words properly while speaking.
I want to build my vocabulary'（ボキャブラリーを増やしたいです）
と、
'I want to grow my vocabulary'（ボキャブラリーを増やしたいです）
と、
'I want a larger vocabulary'（ボキャブラリーを増やしたいです）
は 'I want to expand my vocabulary' と同じ意味です。より多くの言葉を学びそれらを使いこなせるようになることを意味します。
If you want to do something better then as well as 'better' you can also say you want to 'improve' it. If you want to expand something you want to know more and make it bigger (so you know more words)
ご質問ありがとうございます。
I want to 私は～したい
expand 広げる、拡大する、拡張する
improve 進歩させる、上達する
my 私の
English 英語の
vocabulary 語彙、用語数
expandを使うと語彙力がいろいろな方向にどんどん広がっていくようなイメージ
improve を使うと右方あがりに語彙力が上昇していくようなイメージ
少しニュアンスが異なりますが、語彙力を増やしたいといいたいときはどちらの表現も使えます。参考になれば幸いです。
イギリス 
カナダ 
アメリカ合衆国 
日本 
