* おはようございます (Ohayou gozaimasu): This is the most common way to say "Good morning." It's polite and suitable for most situations. While it doesn't explicitly say "sir," the polite form implies respect.
* こんにちは (Konnichiwa): This means "Good afternoon/Good day." It's less formal than "Ohayou gozaimasu" but still polite and widely used. Again, it doesn't specify "sir."
* こんばんは (Konbanwa): This means "Good evening."
* 今日は (kyou wa): This literally means "Today is..." You could use it as a very informal greeting, but adding something like "よろしくおねがいします (yoroshiku onegaishimasu)" afterwards ("Pleased to meet you" or "I would appreciate your help") would make it more appropriate.
For a more direct translation conveying the "sir" aspect, you would need to add a honorific. However, Japanese generally doesn't directly address someone as "sir" like English does. Respect is conveyed through the politeness level of the language itself rather than a direct title.
Therefore, おはようございます (Ohayou gozaimasu) or こんにちは (Konnichiwa) are the best choices, depending on the time of day. They are both polite and appropriate for addressing a man you don't know well or are meeting for the first time.