Hand selection is arguably the biggest element involved in pre-flop play. No matter how perfect your strategy might be, nothing will matter if you don't know which hands to play in which spots. Hand selection is also one of the most diverse yet extremely easy to understand strategy topics in all of poker. The reason for this is that there's a general set of hands that will make the most sense to play, but there's also plenty of scenarios where those same hands will lose a significant amount of their value. Therein lies both your opportunity to make the proper plays and to expose opponents who don't know when to step out of the way. Pre-flop play is largely a solved science, but hand selection isn't as easy as it might seem.
There are many different elements that factor into the equation of proper hand selection.
There's nothing that will give you more of an advantage than the opportunity to act last in a hand.
Are you facing a weak passive player or a loose aggressive player who is looking to donate their stack? You'll need to adjust which hands are worth playing based on who you are playing against.
A short stacked player will demand that you play different hands than when you are playing in a heads up pot where stack sizes are 200+ big blinds deep. Finally, your image at the table will also play into what hands you can and cannot get away with playing profitably. If everyone thinks you only play strong hands, you'll be able to take advantage of this image by playing pots with suited connectors and other similar hands. Likewise, if your opponents feel that you are a super loose caller, tightening up will most likely be in your best interest. All things considered, hand selection is a much more diverse and dynamic topic than most people give it credit for.
Position is talked about in just about every topic concerning poker strategy, and with good reason. When it comes to hand selection, position is almost always the most important factor when deciding which hands will be most profitable for you to play. Take a hand like AT for example. If you are an early position in a full ring game, this may very well be an open fold. If your position is shifted to the button and there has only been one limp ahead of you and no raises, however, AT is now a hand that's worthy of a raise.
Position is going to be most critical when you are playing in a pot that's likely to become somewhat confusing during post-flop play. Suited connectors and small pocket pairs are two examples of hands that play terrible out of position, but also work phenomenally when you are in position. The more speculative your hand is, the less value that it has as you move further and further from the button.
When you have big hands in early position, the action at the table won't be nearly as important. In late position, always be prepared and willing to play more hands. This means everything from K2 suited in a stealing situation to big pocket pairs. If you don't take advantage of your position, you'll slowly be chipping away at profits that you should have had.
Opponents won't always mean a whole lot when considering whether or not to play a hand, but sometimes they will mean everything. If you are sitting at a table that's full of very tight players, you should be looking to widen up your normal range of pre-flop starting hands. On the other end of the spectrum, you should take advantage of aggressive players by waiting for them to self-implode. This doesn't necessarily mean that you should tighten up, but that you should instead look for hands that are most capable of winning an entire stack.
Stack sizes should have a recurring role in your decision making processes, and hand selection is no exception to this rule. When you are working with a larger number of big blinds, you'll be able to play a greater number of hands. Remember, though, that effective stack sizes are all that truly matters. The effective stack size in a hand is the top amount that both you and your opponents(s) are capable of playing for. As an example, if you have $873 and your opponent has $1,023, the effective stack sizes are $873.
This information will allow you, for example, to determine whether a hand is worth playing against a re-raise in a spot where it would normally be an insta-fold. Though not usually the primary element in deciding whether or not to play a hand, stack sizes can be exceptionally valuable in many situations, and especially those were you are looking to set mine, flop a monster, or get out of the hand altogether. You have to have an idea of how much money any hand stands to realistically win.
The easiest way to approach table image as it relates to hand selection is to think about how you can be most deceptive given what you know that your opponents already know to be true. If you have shown down only big hands, start being aggressive with weaker hands. If you have been caught bluffing or have shown down a bluff in a previous hand, start doing your best to extract huge amounts of value out of your made hands. Table image is a great tool in hand selection, but it is key to remember that it's very volatile and is always changing.