終わる is an intransitive verb, so the thing that ends is the subject, marked with が. For example, 会議が終わる means 'the meeting ends.' It is not used with を to mean 'finish something'—that would be 終える (transitive), but 終える is more formal and less common in daily speech. When you want to say you finish doing something, combine the ます-stem of another verb with 終わる, like 食べ終わる (finish eating) or 読み終わる (finish reading). This is a common pattern and works with many action verbs. Note that 終わる can also be used as a standalone verb in past tense: 終わった (it ended). In casual speech, you often hear 終わった used to mean 'it's over' or 'I'm done.'