You are Not Vulnerable, while the Vulnerable opponent on your right deals and opens 1H. You hold...
and I get asked "What should I bid? I was going to open (1H). Should I (A) double or (B) lie a little and bid 1NT?"
The question ought really to be not "what" but simply "should I bid?" And the answer is a resounding NOOOOO!!!
The opponents have bid a suit in which you are strong, so why tell them that. It's unlikely the bidding will end up in a H contract anyway, but careless talk costs lives (and tricks!)
(A) A double in this situation would be "for Take-out." It is NOT a penalty double. It would carry the message that you are happy for partner to bid any other suit than H's at whatever level they deem fit. This is not a message you want to send with this hand at all. PASS.
(B) 1NT as an overcall promises 15 - 17 HCP and a good (ish) stopper in their suit. You only have 12 HCP. Do not lie about this hand. PASS.
(C!) In a perfect, ideal, wonderful world of supreme happiness the auction would - you fervently hope - go like this:
1H - PASS - PASS - DOUBLE!!!
PASS - ???
Partner's double is for take out, as before, asking you to bid one of the other three suits. But before you reach for the 3C card have a think. Say you make 3C that's +110. Say you end up in 3NT and make +400. All fine. Except what if you PASS again? They play in 1H Vulnerable and go 2 down for +500 to you? That's the perfect score isn't it? And that score is only a (remote) possibility if you PASS twice. if you try to bid in these situations you immediately lose the chance for the magical +500. Even 1 down for +200 could be the top score possible. Nobody knows. And nobody ever will if you try to bid with these hands.
The question you should really have asked is "WHY should I bid and WHY should I pass?" 9 times out of 10 my answer will be "What's the score?"
When the opponents bid your suit - PASS, unless you have a hand suitable for a 1NT overcall (15 - 17) If you have an even bigger hand (18+) you can then double as you will re-bid NT's - promising a hand better than an immediate 1NT overcall. On a slightly different note: when you do pass, pass smoothly. Do not dither. Do not hesitate. Do not by your demeanour let them or your partner know that you have this type of hand. If you do so you give the game away to the opponents, which is pointless. OR you give partner unauthorised information which is unethical. Be careful about this. In general try very hard not to hesitate and then PASS. To hesitate and bid is fine.