Of course, save before big fights, such as the fight against Snorlax. Heal your Pokemon at a Pokemon center often, and save up as much money as you can. There are a LOT of things you can buy in the game, some only at specific places or Pokemon centers. Your attire is mostly for show and fun, but some cafes won't serve you if you don't wear certain attire (these are specific to Lumiose City and are almost all optional. It is never required that you dress up for anything necessary to complete the main game). There's also a certain sidequest that involves wearing specific attire, but I haven't bothered looking up how to complete it quite yet.
Get as many TMs as you can. They're hidden all over the world, and sometimes require you to use HMs to find. TMs in this game can be used as often as you like. They aren't single-use. But HMs still require deletion before your Pokemon can learn new moves.
Also, some HMs aren't required at all to beat the game. I recall that I never had to use Cut throughout the game to get to the Elite Four.
Also, you can fight Xerneas all you like before catching it. The other legendaries and 1-shot Pokemon in the game you cannot, however (so make sure to save often!).
Also, you will get a starter after you beat the Elite Four, one that your starter is super effective against. So since you chose Fenniken, you will get Chespin.
Super Training is very helpful. It can be somewhat bland, but is a good way to power up your Pokemon quickly. Pokemon Amie is important for one particular spot in the game. This spot teaches your Pokemon the most powerful move it can learn if your friendship levels through Pokemon Amie are high enough.
Use O-Powers often, even if you don't have a need for them at the moment. They will level up and be more useful later. Also, visit hotels when you are in the area. The O-power guy will give you a new O-power if he's there. I believe he's dressed in pink, so it's not too hard to recognize him.
To find the mega stones later on in the game, you might want to look up a guide. You can only find them after beating the Elite Four, with the exception of Lucarioite and the mega stone of the Kanto starter you choose later in the game (I believe you have to buy the stone somewhere in Lumiose City, but I may not remember correctly).
Hope this helps!
Of course, save before big fights, such as the fight against Snorlax. Heal your Pokemon at a Pokemon center often, and save up as much money as you can. There are a LOT of things you can buy in the game, some only at specific places or Pokemon centers. Your attire is mostly for show and fun, but some cafes won't serve you if you don't wear certain attire (these are specific to Lumiose City and are almost all optional. It is never required that you dress up for anything necessary to complete the main game). There's also a certain sidequest that involves wearing specific attire, but I haven't bothered looking up how to complete it quite yet.
Get as many TMs as you can. They're hidden all over the world, and sometimes require you to use HMs to find. TMs in this game can be used as often as you like. They aren't single-use. But HMs still require deletion before your Pokemon can learn new moves.
Also, some HMs aren't required at all to beat the game. I recall that I never had to use Cut throughout the game to get to the Elite Four.
Also, you can fight Xerneas all you like before catching it. The other legendaries and 1-shot Pokemon in the game you cannot, however (so make sure to save often!).
Also, you will get a starter after you beat the Elite Four, one that your starter is super effective against. So since you chose Fenniken, you will get Chespin.
Super Training is very helpful. It can be somewhat bland, but is a good way to power up your Pokemon quickly. Pokemon Amie is important for one particular spot in the game. This spot teaches your Pokemon the most powerful move it can learn if your friendship levels through Pokemon Amie are high enough.
Use O-Powers often, even if you don't have a need for them at the moment. They will level up and be more useful later. Also, visit hotels when you are in the area. The O-power guy will give you a new O-power if he's there. I believe he's dressed in pink, so it's not too hard to recognize him.
To find the mega stones later on in the game, you might want to look up a guide. You can only find them after beating the Elite Four, with the exception of Lucarioite and the mega stone of the Kanto starter you choose later in the game (I believe you have to buy the stone somewhere in Lumiose City, but I may not remember correctly).
Hope this helps!