I want to make a character that is a rogue scientist (artificer is not something that I want) that uses his knowledge of elements to make fireballs or home made explosions. How would  I do this?

Take a look at this article from Tabletop Builds: Flavor is Free: Tasteful D&D Reflavoring.

Basically, you can take anything in the game and reflavor it. If you want to have a scientist who makes fireballs using science, make him a wizard who casts Fireball, but the "magic source" is science. As long as you do not change any of the actual game mechanics, you are still playing D&D.

Edit: Oh you mean a Rogue who is a scientist, my bad. I was thinking of the adjective rogue.

Alchemist Supplies

+

Gunpowder

.. Enjoy.

Well, the thing is: why do you need it to be a rogue if you're going to spam fireballs anyway? Depending on how you answer that question, you might have some good alternatives! Feats, for example. 

If you need expertise for that science feel, you can take the Skill Expert Feat.

What level is the Rogue?   At 13th level options open up.  A 13th Arcane Trickster can cast a fireball, and a 13th Thief can use wands, including a wand of fireball.

Just relabel it as science and boom, done.   Sort of like taking a swords and sorcery world and calling them Light Sabers and The Force.

I suppose flavouring it as two separate chemicals in a bottle when thrown can do this. I immediately thought of the gum in mission impossible one as possible inspiration. 
But then again, for me that would cease to be magic and venturing into artificer territory.  But other responses above are valid, it’s a matter of reflavouring it. 

You want alchemist's fire.

I like the idea of being an Arcane Trickster and just reflavoring all your magic as a form of science.

I think that's one thing I partly dislike about Artificer... once Artificer was introduced as a class suddenly it sets people to think that, if they want to be a technology-based character in some way, then they need to be an artificer. I think the same thing of the Assassin subclass of Rogue... most people who pick that subclass do so just because of the name, and they feel like if they want to be a cool assassin character then they obviously need to be the assassin subclass, when really any character could be an assassin if they take the right background.