Only
Denotes exclusiveness: "only (...)" だけ is the antonym of しか, but as しか can only be used in the negative, they are used similarly.
1時間だけ寝た I only slept one hour.
お母さんにだけ手紙を書いた I wrote a letter to only my mother
魚だけ食べなかった I only did not eat fish
As (...) as possible
when used with the Potential form (〜eる), it denotes that you do something to a degree of which you are able to do it. Fittingly sometimes referred to as the 可能型.
ランドセルに荷物を入れられるだけ入れて、家出した。 I put as much stuff into my randoseru (backpack) as I could, and left the house.
Very commonly used set phrase is できるだけ
今日は息子の誕生日なので、できるだけ早く家に帰りたいです。 Because today is my son's birthday, I want to go home as soon as is possible.
As many/much as possible
when used with the 〜たい, or another verb that denotes wanting (好き、欲しい) denotes that you want to do something as much as you want, (usually unlimited, you glutton)
日本に帰ったら、すしを食べたいだけ食べたい。 When I get back to japan, I want to eat as much sushi as I can possibly eat.
好きなだけ持って行ってください。 Please take as many as you like.
To try to do as well as one can
very similar to 〜て見る. ダメかもしらないが、〜してみる. Repeats the verb.
合格できるかわからないけど、受けるだけ受けることにした。 I don't know if I can pass but, I will try to take as much of the test as I can
開いているわからないけど、行くだけ行ってみよう. I don't know if it's still open, but I still want to try to go!