How do i attack with a weapon ? and How do i switch the weapon from your lets say right hand for your weapon in your left hand ?

To attack with a weapon, get your weapon out, hold down Left Mouse Button and drag your mouse towards the direction you want to attack.
You can make left-right, diagonal, thrust and up-down attacks.

For switching weapons, I've configured my controls to have it by pressing Y...
To know what the default key is, open the control configuration in the game's menu.

Eh.. By default, right mouse button. From personal experience, I might add that this interface is interesting if you have a wireless mouse and your batteries aren't fresh.. *heh heh*