Just like in the previous installments of the game, Batman has a very wide array of skills and gadgets used not only for winning fights, but also to eliminate enemies stealthily, and for reaching inaccessible places where, usually, valuable items are hidden.
Some of the skills are available right from the beginning of the game but, many of them need to be unlocked first. You can do it in two ways. Close combat upgrades and invisible predator upgrades are unlocked by means of skill points that are awarded each time the Batman reaches the next experience level. I recommend that you search for opportunities to collect large amounts of points by, e.g. doing well in combat, finding collectibles or by completing side missions. This will allow you to unlock better skills quickly and make the game easier. The auxiliary upgrades are also inaccessible, by default, but they are different from the two groups mentioned above in that they are gained in a different way, e.g. by completing the side challenges, or by finding collectibles of a given type.
A very important piece of information is that you can review your gadgets at any point throughout the game. It is a good idea not only to remember the correct button combination, necessary to initiate another attacks, but also to plan on your purchases because, you know very well what a given upgrade can offer.