Hello and welcome! Nice start!
Regarding the noise/grain, it's because you are rendering with too few samples. You're going to need quite a bit more than 100...
In general, the more lights in your scene, and the more intense the light, the grainier. But the types of shaders you use can also add noise; for example the Glass and Glossy BSDFs.
To counter this, you need to increase your number of samples at render. Adjusting the Clamp also helps. Do a few test renders, progressively increasing your samples, to see if the noise starts lessening, though it is going to take longer to render keep in mind.
For mesh deformation... I personally can't tell from the render. You can show us your model topology, to see if we spot anything, and show us your rig with the weights
Hope this helps a bit and keep us updated on the progress!