If you cannot take or build, you must discard.
A discard is placed as a single card in the centre, not on top of other cards.
Always restore your hand to five cards after every turn.
This is the only way the cards in the centre are replenished (to be used in later takes and builds).
You may discard any card from your hand. However, where possible, plan ahead and discard a card you could use on your next turn.
One of the first things to look for when discarding is a number sentence. With your 4 and 5 adding to 9, you would discard the 4 or the 5 (usually the higher of the two numbers you wish to build). If it is still there for your next turn, you could add the other number to it for a build of 9, which can be taken in the following turn.
The second option for discarding is to look for a matching pair. Having two 12s, you would discard one of them.
If it is still there for your next turn, you could take with your remaining 12.
If none of the above opportunities present themselves, or if you are in doubt, it is suggested you discard a low card.