I wanted to make some molotovs so i bought a fuel schematic, learnt it and when i try combine the yeast and wine to make fuel, it tells me i lack the expertise, i dont get it, i learnt the schematic and cant find "fuel" anywhere on my character sheet under explosives or anything

It won't be on your character sheet. It's in the Learned Schematics pages only.
That said, it means you don't have enough skill in whatever those components is to make it. Whereas the schematics which you learn as a consequence of your character leveling only require one scientific discipline (for example, featherweight chainmail requires only skill with leather armor and such), Learned Schematics can be weird. Fuel's one of the less bizarre examples.

Lemme give you an example. Under the Weapons learned schematics, you'll likely encounter some day an Envenomed Sword. This will require the Smithing school because it's a sword, but it will also require the Chemistry school because it uses poison.

Fuel's not so bad. A site online says it requires Chemistry Level 3, whereas the Envenomed Sword requires "Chemistry Level 6 *AND* Smithy Level 1." But the weird factor's pretty straightforward here: You have a schematic under Explosives which requires Chemistry. If you don't have much expertise in Chemistry, you have to buy manuals in Tarant, learn Chemistry with character points, or give up on using the schematic.
I usually buy manuals.

Found schematics (as opposed to learned schematics) require certain tech skills to use, sometimes from more than one discipline as MackieStingray already mentioned. However, you don't need to look online to find these requirements; they are listed on the schematics themselves.

If you look at your Fuel schematic, you will see pictures of the ingredients you need, but in the corner of each picture there will be a small symbol, and in the other corner a number. The symbol tells you which tech discipline you need skill in, and the number tells you how much skill you need. For Fuel, you should see what looks like a potion in the top left corner of the ingredient box; this represents chemistry. The number probably reads 30, or maybe 25 (definitely more than 20 though).

You get 10 points of skill for each level in a tech discipline. But, you don't have to level up a tech discipline to make this stuff; you can instead buy tech manuals. Tech manuals are available for each tech discipline, and if you are carrying one, it will grant you a level of skill in that discipline equal to your intelligence stat. So if you have an intelligence of 15, you will gain 15 points of skill in that discipline for carrying the manual. BUT, you can carry multiple manuals, and stack the bonus. So two chemistry manuals would give you 30 skill, if you have an intelligence of 15. With enough manuals you can usually make anything even if you are unskilled in the discipline(s) required.

You have to be carrying the manuals at the time for the bonus, and they are heavy, so it's advised to find a safe place to store them when you are sure you won't need to craft things. For Fuel, you probably want to make it a lot so you might want to lug the manuals with you, but for things like weapons or armor you may only want to craft it once, then drop the manuals somewhere.


***MINOR SPOILERS***


To see what I am describing, here is a screenshot of one of the found schematics in the game:

http://lparchive.org/Arcanum/Update%2034/LPA34-55.jpg

Here you can see that there are two ingredients, one shows a potion representing chemistry, and the number says 55, meaning you need 55 skill in chemistry to make it. This either means 6 levels in chemistry (60 points), or using tech manuals to get your skill bumped to 55, or a combination of actual skill and tech manuals.

The second ingredient shows a lightning bolt representing the electrical discipline. The number reads 40, so you need level 4 (or tech manuals) in Electrical to make the item.

In total, 55 chemistry and 40 electrical will let you craft this item.


***END SPOILERS***