どうせ is a 副詞（ふくし） (adverb) used in Japanese to express that some particular state or condition of (A) will result in (B). Usually, there is no way to stop (B) from happening, and it is considered a natural consequence of (A). In English, this translates closely with phrases like ‘at any rate’, or ‘in any case’.
Being an adverb, どうせ most often appears at the beginning of sentences, before going on to describe to inevitable situation.

• どうせまたパチンコに行（い）くんでしょ。
  In any case, you are going to the pachinko parlor, aren’t you?
• どうせ私（わたし）のことなんて好（す）きじゃないんでしょ。
  In any case, you probably don’t even like someone like me.

どうせ often carries the nuance of frustration or resignation that (A) cannot be changed. In these cases, it is often paired with other grammar structures such as なら or だから. This shows that if (A) cannot be changed, then (B) is a good idea. This usage translates similarly to ‘if (A), you might as well (B)’.

• どうせ新（あたら）しいスマホを買（か）うならもうちょっといい奴（やつ）買（か）いなよ。
  If you are going to buy a new smartphone, you might as well buy something a little better.
• どうせまた負（ま）けるんだから頑張（がんば）っても時間（じかん）と体力（たいりょく）の無駄（むだ）だよ。
  We are going to lose again anyways, so trying our best would be a waste of time and energy.