Originally Posted by: ingridguerci94 
but i completely lack any knowledge about 3d modeling and everything related to it.
now i want to figure out if i should try to gain said knowledge or if i should just buy all the stuff i want.
So.. first: 3D modeling isn't just about one single activity but actually consists of a multitude which all involve 3D figures. For example... I'm a vivid user of "Daz Studio"; this is a 3D modeling program which allows me to place 3D figures in a virtual scene, apply certain poses to them and then create an image from that; this is also referred to as rendering: the computer builds up the image based on the behavior of the different assets. For example: a metal ball generally reflects much more light than one made out of cloth.
The 3D figures I use are part of my library which I've build up over the years. Aka: bought / grabbed from different places. Models of humans, animals, aliens but also so called "props" like tables, chairs, etc. To get a bit of an impression just skim the Renderotica store aka the catalog.
Then we get to the second example... 3D editing / sculpting. In other words: instead of working with existing figures you're creating your own, or edit existing ones. Now we're talking about programs like Blender, Hexagon or ZBrush (there are more, but these are the ones I'm familiar with).
Which brings us to your question...
Originally Posted by: ingridguerci94
I need 2-6 base models (one male and female, perhaps splitted into caucasian, asian and black "races", if that would make sense).
they should be game optimised and, if possible, made into an UMA.
This is of course just my 2 cents, but I'd say your most viable option is to purchase the assets you need. For the simple reason that I don't see someone who is completely new to this whole thing "just" creating some human figures out of the blue on their own. The creative process is anything but easy.
Which brings us to yet another important detail... if you are going to purchase 3D assets to use them in a game then make very sure that their license allows for this. Context heavily applies here of course, but generally speaking a regular license doesn't cover this: you're probably going to need a so called interactive license. It's all in the name really: a regular license is personal: it allows you to use and interact with the product to create images (or videos).
But when you use 'm in a game you're basically allowing other people to interact with them. As such... it requires a more extensive license.
Of course this doesn't apply to all 3D assets, but it's definitely something to keep in mind.
Hope this can give you some ideas.