Opening the bidding with 1NT.
If you have 15-17 HCP and a balanced hand ALWAYS open 1NT. If you have a second suit to rebid, you may open 1 of a suit. If you open 1NT with a 5 card Major, learn the conventions for managing that situation.
The risk in not opening 1NT with 15-17 HCP is that you can miss a 3NT game if partner has 8-9 HCP, because when you rebid 1NT with 15 HCP but without a second suit, partner will pass. You will be left in a 1NT contract making 9 tricks for a bottom board!
Some people think that they need to have all suits stopped, but that is wrong thinking. When you open 1NT your partner has a complete picture of your hand in one bid.
Remember you only need 7 tricks in 1NT. Even if the opposition run the first 5 tricks in a long suit, you can still make your 7 tricks.
If you can't make your 7 tricks it usually means you have stolen the contract from your opponents and should end up with a fair result anyway.
Advancing partner's overcall
If you do not have sufficient points for game over partner's minimum point overcall, consider passing as your first option.
If you must bid, a good rule of thumb is to subtract 2 from your hand value and then consider bidding as though your partner opened.
If you Advance with 1NT it should tell your partner that you have an opening hand with stoppers in the oponent's suit(s), but without a fit in partner's suit.
Only bid your own suit over partner's overcall if you have good point count, are short or void in partner's suit, and have a long suit of your own.