«Ставка» is a noun, so «я ставка» would mean "I am a bid"! According to MasterAdmin's translation, "I bid" (past tense) would be «я сделал/а* ставку» (literally "I made a bid"), and "my bid" would be «моя ставка».

Bear in mind that "I bid", in English, is ambiguous, as it could be either simple present or simple past tense ("I bid on auctions every week", or "I bid on an auction last week".)

* «Сделал» if the speaker is male, «сделала» if the speaker is female.