I, as a GM and player really love games that focus on total war, combined arms, and heavy use of clever tactics and strategy, A major part of any military operation is artillery, whether it is indirect or direct fire. As far as I have found however there is absolutely no rules about it written in the core rule book, and mention of artillery in the supplemental books i have is almost nonexistent, any suggestions?

Here's what I would suggest for indirect artillery:

It uses Gunnery with the Intellect characteristic, rather than Agility. I imagine most people who have operated artillery or mortars would agree with me on that.
It treats cover as you would if you were firing from an elevation (however it is you treat that). Against targets of sil 2 or smaller, it gains the Inaccurate +1 quality.
Probably give it a minimum range of Extreme on the Personal-scale.

As for statting it, most of that is easy enough. Breach and Blast are pretty much necessities, Slow-Firing makes sense as well.

If you're using physical shells, then you can have different varieties, like high-explosive, armor-piercing, or frag. From there, you can mess with Blast radius and rating.
Just off the top of my head, I'd say something like this:
AP shells: Damage 6; Crit 2; Blast 1-Engaged, Breach 4.
HE shells: Damage 8; Crit 3; Blast 6-Short.
Frag shells: (Personal-scale) Damage 20; Crit 2; Blast 15-Short, Vicious 3.

The other stats (Slow-Firing, Inaccurate, range) would be given to it via the weapon itself. Different varieties of artillery would have different ranges and levels of accuracy. An easy way to adapt shells to different varieties of artillery would be to increase damage, Breach, and maybe Blast by a set number for each level of artillery. For example: Tier I would have no modifiers, Tier II would increase damage by 1, Tier III would increase damage by 2 and Breach by 1. Any artillery that falls within those categories adds these modifiers to the shells it fires.

For direct artillery, pretty much any planetary-scale weapon is just that.

This also depends on whether you want players shooting the artillery, or being shot at by artillery. In my games, artillery is either very abstract (through mass combat rules), narrative (if the players need the artillery for a single purpose, such as destroying a specific target, or they call in an NPC barrage), or as an environmental hazard (any area under bombardment is treated as difficult terrain, requiring coordination rolls to get through. Difficulty and modifiers based on size and type of barrage. Damage based on threats, failures, and despairs). I haven't had any players interested in operating indirect artillery for any substantial amount of time, although they like being able to call for fire support.