たらいいですか

たらいいですか asks what you should do: "what should I…?", "where should I…?". A question word comes first, then the verb in the たら form and いいですか – literally "if I do …, will it be good?".

You are asking for instructions from someone who knows better. Among friends, ですか drops off.

たらいいですか or てもいいですか

Without a question word, たらいいですか checks an instruction: "is this what I do?". It does not ask permission. For "may I?", use てもいいですか.

• × 窓を開けたらいいですか (for "may I open the window?") → 窓を開けてもいいですか

Politer and plainer

To give advice rather than ask for it, use たほうがいい or たらいい.