How to reveal an object ?

You need to spend some serious time with the learning resources that you can find in the Learn section of the After Effects Home Screen or on the AE product page. Shape layers are created with the pen tool (g), or with the Parametric shape tools: Rectangle, Ellipse, Polystar, and others. 

Here's a link to the learning resources: Learn AE

I would start a shape layer, then create a path with a stroke only that you can animate using trim paths. The path is drawn so you cover the parts of the image that you want to reveal. Then you apply Trim Paths to the shape layer and animate the path to reveal the layer below. The last step is to set that shape layer as a track matte. 

You'll find more info here: Learn AE

Depends what you mean by "little by little".

If you want the area inside the mask to gradually appear starting in the middle, then just keyframe the mask's Expansion parameter, going from a negative number towards zero.

If you want the area inside the mask to appear in random pieces, then you can luminance mask your already-masked layer with a second layer that contains something like a fractal noise effect, and keyframe its brightness slider.