From what material/s I need to create a mold so that I can pour in it safely any kind of molten plastic?
Injection molding can in fact be done at home. But judging from the OP this method would be far and above beyond the scope of his current knowledge on these types of things. 

The easiest way to cast plastic parts at home or in your workspace if that’s the case would be with the use of a commercially available product such as SmoothOn. 

Simple. Sold on amazon as well as most other online retailers. 

I suggest doing at the very least some cursory research into the process’ used when casting/molding with SmoothOn products, as they offer a variety of materials such as polyurethanes, rubbers, silicones etc. 
a quick google search will lead you in the right direction. YouTube aswell. 

Also, just as a quick tip to lead you in the right direction, try finding some videos of sculptors and character designers using this stuff for they’re own casting/molding needs and you’ll have a good point of reference to start from. 
Good luck :)