I have no idea how to start with mods

Hi. You can start from things that are the easiest: custom eyes, eyebrows and face/ whole body skin overlays. You will find many tutorial that should not teach mistakes because there is not much you can mess up here. Most of them will be for Photoshop, and if you do not have it, you can use Photopea, an online Photoshop rip-off. It is almost identical to Photoshop, so you should have no problems following the tutorials.

The second thing i recommend you  is to check pose making because this is one way of making sims taller (by changing the rig). This effect is not permanent, though, so you may want to look into sliders. s4s cannot be used to make sliders, there is a different software called s4 morphmaker for this. You can also use it to create more realistic facial features presets.

I assume you're talking about the alpha hair and it is problematic to make. If you really really want, I'd suggest you to start from editing maxis hair (to wrap your head around blender ui and how to adapt the hair for ts4), then you can try frankenmeshing and adding small details ( to learn how to model, unwrap, texture hair). Once you'll figure that out, you should try converting a maxis hair to alpha (not going for something pretty, just adding textures with transparency ). You will encounter the problem that makes the alpha hair troublesome to make and will need to study other people's works to understand why they split theirs the way they do. Once you'll figure it out, you should be able to make your own hair with no problems.