There is no standard. About as close as you get to one is using the left stick to move and the right stick to look around or aim and using B for Cancel/No options (but no general standard for the Accept/Yes option!). Other than that it's all over the place.
A number of popular modern games have used all of the following for a primary fire button:
- A
- X
- Right trigger
- Hold left trigger and then hit one of the above
Note that right-trigger is very popular for shooters simply because it allows you to use both sticks (move and aim) while firing. Games that don't use the right-stick for anything in combat are more likely to use face buttons for attacking.
For secondary/melee, I've seen:
- A
- B
- X
- Y
- Left trigger
- Hold left trigger and then hit one of the above
- Tap right trigger
For opening a mini-map, I've seen:
- Y
- X
- B
- Menu
- Start
- D-pad (several different directions)
If in doubt, just go play a couple games in a similar genre to your game and see what they did. See if you like how it feels.
However you do it, do your in-game help properly! When a user is first expected to do something, tell them to do it and show them how. Don't just put some text that says "Press A to shoot." Pop up a picture of the A button (green circle, A in the middle) with "SHOOT!" next to it. If it makes sense in your HUD, keep the buttons displayed next to equipped items, so the user can instantly see what to press to use something he has picked up. If the usage is even remotely complicated or non-obvious, have some kind of tutorial (preferably in-game, not a popup/video!). Good tutorials are generally similar to ones where the user is stuck until he uses/masters the new move/item, with some kind of in-game reminder of "press [button image] to [action]" messages until he escapes the area.
Point being, you should be more concerned with ensuring your game can teach your player how to play it than hoping that you can rely on the player intuitively knowing how to play your game. Even if there was some standard-ish layout, you want to support new and casual players as well as hardcore gamers.
A wise man once told me that all games are basically just really long tutorials on how to play that one game. While I'm not sure that's true for every genre or type of game, it's a very good rule of thumb to keep in mind.