だけ
only, just
Noun / amount / verb + だけ
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だけ means "only" or "just", and goes right after what it limits: 水だけ飲みました, "I only drank water"; 少しだけ, "just a little".
It can follow nouns, amounts and verbs: 見るだけです, "I'm just looking", is the phrase to know for shops.
が and を can drop after だけ or stay after it (水だけを飲む); other particles usually go after it: 私だけに, "only to me".
しか, a later point, also means "only", but it needs a negative verb and sounds like "nothing but, not enough": 水しか飲みませんでした. だけ is neutral.
Examples
昨日は水だけ飲みました。
Yesterday I only drank water.
日本語は少しだけ話せます。
I can speak just a little Japanese.
一つだけ質問があります。
I have just one question.
大丈夫です。見るだけです。
It's okay, I'm just looking.
このことは私だけが知っています。
Only I know about this.