CaioM you are doing it all wrong.
First, no, it's not simple. Back in the day games didn't show your legs.
1 - A first person model MUST use different animations than a third person one. Try looking for first person animation from AAA games to see what they look like. A good start is Mirror's edge, there was a video showcasing the player animations. As far as I'm aware, you HAVE to create these yourself, animate the character while looking at it from the camera.
2 - The character must be in separate meshes, Mirror's Edge could be the exception since you don't use guns (I think). Look at CS animations, or from any FPS. The arms and the gun are it's own object, the camera comes from the chest and they are positioned around it.
3 - I don't know what your character code looks like, but the camera must be parented or follow the Character3D, so when you rotate in Y the arms and legs and camera follow the Character3D. There must be a Node3D at the center and the arms and camera must follow it. Like this:
Character3D (rotates in Y)
|
|->Node3D (rotates in Z)
| |->Arms
| |->Camera
|
|->Legs
The problem with using a complete character is that the camera will clip through it and look all wrong, specially the head and chest, this is because these move all the time.
Some really bad unity tutorials would tell you to put the camera in front of the face. This is a bad idea. It leads to either an unstable and nauseating camera, or excessive clipping.
4 - Finally, play around with camera near distance and change your visibility layers. There must be ONE layer that's not visible to the First person camera, and one that is ONLY visible to the First person camera.
Lets call the first one "Third person". You put here things that the First person camera should not see. You can put a fully animated third person character here, so it can be seen in mirrors. It can also maybe be used to cast shadows.
We'll call the second "First person". Only the arms and legs will be in here. this way you don't see the shadow of your two floating arms, and they don't show up in other cameras.