I was fighting with a scimitar and suddenly I noticed I'm actually fighting with a common sword. How do I drop this weapon and pick up a guards weapon? I wanna pick up an axe or spear.
Some guard must have knocked the weapon out of your hand and you grabbed another weapon.
There are two ways to acquire another weapon: the first is to get one from the ground, after you have killed its owner. You must switch to Unarmed (down on the D-Pad) and press the pick-up button (Y or Triangle, IIRC).
Another is to disarm your enemy. Using Unarmed, you must perform a counter-attack (RT+X or R2+Square) when your enemy attacks you. Note that boss enemies can't be disarmed and some attacks can't be countered.