Whist is a team game, so in order to win you must communicate with your partner and be supportive of her choices. Partner games can be tricky, so pay attention to what your partner is playing and follow accordingly. If you win the bid, throw out your first card from the suit you are strongest in. This will be a signal to your partner. If you are not the player to win the bid, a way to signal to your partner that you are strong in one suit is to throw out your lowest card in that suit when you are unable to follow another suit. This shows your partner where you will be able to help. You must also pay attention to the first card thrown out by your partner so you know where his strength lies! Unfortunately, if you have no strengths and play a card, your partner might assume that you're indicating strength when you aren't.
If your partner is leading a trick, you'll want to follow his lead. If he plays a high card, you'll want to play your lowest card — it doesn't make sense to waste his high card by playing one over it. If he plays a low card, you'll want to play a high card to win the trick.
If you win a trick, you'll want to play all of your winning cards before handing the lead back to your partner. You'll also want to try to pick up all of the trumps before playing an alternate suit. This allows you to determine what remaining trump cards there are in the game.
If you are playing a no-trump hand in Bid Whist, pay close attention to the cards your partner is playing! If you are not going to try to win the trick, play your lowest card in the suit led. There's no sense in throwing away cards that might be useful to you later. An exception to this rule is that if you only have two cards left in a suit, throw the high one first and the low one second. This is a signal to your partner that you are out of that suit and can now play trump.