〜が上手・下手
good at, bad at
Thing + が + 上手 / 下手 + です
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上手 (good at) and 下手 (bad at) work exactly like 好き: they're な-adjectives, and the skill takes が. 田中さんは料理が上手です, "Mr Tanaka is good at cooking".
上手 is praise, so it's for other people. Calling your own skill 上手 sounds like boasting. About yourself, say 得意 (a strong point) or play it down: まだ下手です, "I'm still bad at it".
That's why 日本語が上手ですね is the compliment every learner hears, and まだまだです ("not at all, still far to go") is the modest reply.
Before a verb, they take に: 上手に話す, "speak well".
Examples
田中さんは料理が上手です。
Mr Tanaka is good at cooking.
私は歌が下手です。
I'm bad at singing.
妹さんは絵が上手ですね。
Your sister is really good at drawing.
日本語が上手ですね。
Your Japanese is very good.
私はまだ日本語が下手です。
My Japanese is still bad.