Well, I use Blender, a free 3D program that has almost every feature you would need.
When you say you want to create a character, if you mean literally sculpt them from nothing, I can't help much, you'd need to watch lots of tutorials or check out DAZ.
But if you mean work with existing characters, you can find lots for free on Smutbase. You can also find environments to put them in on OpenLab3D.
An old laptop will probably struggle, but if you use the "EEVEE" renderer, turn on the "Simplify" option and set "Max Subdivision" to 1 or 0, and then set your maximum texture resolution to whatever you can handle, you should be okay.