There are ten stone lions in Hogwarts. To get them, you have to search for a student near every single one of them and he will give a mission. Most of times, it will be winning a duel with a student bullying someone. Some of them will ask you to prepare them a potion which you can make in the Potions Club. After completing the task, you will be able to take the crest from near the lion. The students are placed randomly, so search for another one if there's no one around. Below there's a description of the places where you can find them.
The first lion can be found near the forest behind Hagrid's Hut.
The second lion can be found in the corner of the Courtyard Entrance.
At the bottom of the Clock Tower, in the corner of the room with the pendulum.
On the Training Grounds, going right from the Potions Club entrance. The lion is in the corner at the end of the wall.
In the corner of the corridor by the Transfiguration Courtyard entrance.
At the end of the corridor on the Transfiguration Courtyard.
In the corner at the Suspension Bridge.
In the Astronomy Tower after getting down from the stairwell.
In the corner of the Library.
In the corner of the corridor leading to the Hospital Wing.