I'm not yet confortable to use highlights and shadows to give texture, so I wanted to ask how can I improve the texture of the character ?, does the effects make sense ? are at the right place ? knowing that the light comes from top corner left. thank you :)
Seemingly closed due bad asking. But I think you earn some answer, The shading is consistent altough not top notch perfect Because there's only that one figure or exactly a part of it, I have no idea, is the whole thing also consistent
EDIT: Do not reduce any highlights. The light gradient on the shirt, stomach and pants shoud be directed more left to right to make it cover the left side of the shirt and pants. There should be the same shadow between the legs than there is at the right side.
About the terms: texture = details that make differences between the materials. Shading = how the light and 3D geometric forms have caused color differences (=highlights, shadows and glosses). Gradients are fine for shading smooth forms as your shape.
A little more 3D shading to shirt and pants. Also the curvature is added because everything cannot look out horizontal. I left the sleeves horizontal, so the belt and the bottom of the pants must be bended. Sorry for the coarseness. It's unavoidable in low resolution.
For head consider to use circular gradients. That makes the shading to conform with more probable light conditions. They can be stretched to elliptic, too as you can see, if you look carefully at the muzzle.
EDIT2: Starts to get complex.
I did the recolorings and bends to your image in Photoshop. There's Edit > Transform > Warp for bendings and stretchings. The selected area can be distorted. Recoloring was made by selecting an area and flooding a new gradient. The shade between the legs is coarse due the fast manual brushing into a new layer and thinning with the eraser.
You work seemingly on Illustrator. I recommend to redraw the belt with the pen tool, which is the most powerful drawing tool in Illustrator. Difficult for a beginner - absolutely, but it really pays off to catch that tool!
Alternatively you can add a couple of anchor points, transform them to smooth (by default added points are corners) and resape the belt with the direct selection tool. Third option is to use Envelope Distortion (=several variations available for different complexities)
Chubby belly: It needs only a wide enough gradient and compatible curved cloth edges - as long as you accept body-wide chubbiness. If you want a smaller hill, you increase the complexity radically, A simplified example:
The cloth curves are not adapted, they are still for simple chubbiness.
All gradients are three stop versions to get more control. Top notch shading (by Disney & others) is so complex that it would need Gradient Meshes to be achievable in Illustrator (or alternatively high quality manual painting or 3D modelling). This is beyond the scope of this answer.
The belly consists three layers. At the bottom there's one for the general roundness. On it there is the shading for the separate belly hill. It's selected, so you can see the settings in the different panels. The add-on is partially transparent three stop radial gradient with blending mode Soft Light. The gradient for this purpose is often grey, but it reduces easily the color saturation too much, so I took brown. The belly button was left too weak, so I added a little blurred brown ellipse on the top.
Complex cloth curves: Here the borrom edge of the shirt is adapted to the new belly