I want to show user shot power in standart way (the more user holds finger on the screen the stronger is shot)
Where is the best place to show this icon on touch screen(when I use mouse it's better to show it at the mouse position):
Will be glad to see any good examples you liked.
There are pros and cons to both, but the decision is ultimately yours. You'll have to decide based on the style of your game and the type of game.
Static position: Always in the same place, predictable. This means the user will always know where the indicator will be and allows them to avoid covering it up on their own. Kind of boring and requires some kind of constant HUD.
Dynamic position: Potentially more intuitive, and feels more interactive. Easily allows for a HUD-less display. Potential of being covered up by fingers.
There are changes you can make to both to offset the pros and cons. For example you can make the static indicator pop down when activated, but always pop down to the same spot. Or you can make the dynamic indicator large enough that it couldn't be covered up.
User interfaces benefit greatly from play testing. It's often hard to tell how people will want to use the application until they start using it. For these types of play testing, give it to people and leave the room, so they can test it without you watching over them. Try giving some people instructions and let others try to figure it out on their own. Have a few different versions of the UI implemented for them to test.
As Byte56 said, the decision is yours. But my personal opinion is that an floating HUD game is better. I think something like this:
is better if you don't need the extra space around the user's finger that's occupied by the GUI.
Then you can make it adapt depending on some variables, like it becoming smaller when an enemy is hit, ...
Going to throw another thought out there; how about a ripple circle outwards? Have a circle s that its orgin begins at the point that user touches. Have the radius of the circle expand outwards to indicate that it is "growing" and gaining more power. This will allow the user to see their power gauge immediately as their eyes are already following the mouse - and it gives immediate feedback upon inspection.
Keeping the gauge off to the side is an option if real estate is direly limited but otherwise it keeps the users fixed without having to move around. If aiming is involved, this is especially important; you wouldn't want the user to have to divert his/her attention between two different places.
