If we're looking at truly realistic space combat, let me start by saying that ANY battle would be dictated by orbital mechanics. This would make it extremely difficult and complex for several reasons, and I'll try to illustrate those in my next few paragraphs.

First, let's just say, for instance, that the speed required to maintain a perfectly circular orbit (zero eccentricity) around a planet at 1000 kilometers in altitude is 5 kilometers per second. From here, there are a few things that need to be taken into account. Any ship at that same elevation will have the same orbital velocity, but if their inclination is different (say ship A is orbiting above the equator and ship B is orbiting with an inclination of 2 degrees), then there will be exactly two points where the ships could possibly intercept one another, and that is the ascending and descending nodes (the points at which their orbits intersect). Now, since both ships have the same altitude and with zero eccentricity, the only thing to take into account is their position along their orbital trajectories, and oh shit, ship B is on the other side of the planet.

Now if ship A wants to intercept ship B (let's just say ship B is either adrift or unaware of ship A's presence), ship A is going to have to go into a higher orbit in order to attain a lower orbital velocity by burning away from the center of the planet (the same can be applied in the opposite direction to speed it into ship B's orbit, but would be less efficient as it would have to use more thrust to counteract the stronger gravitational force). This will raise the altitude of their orbit, but also curve it into a parabolic orbit, so another burn will have to be executed once the ship reaches it's intended altitude to compensate for it's new orbital eccentricity. By the end of this, ship A is at 1200 kilometers. Now it's moving quite a bit (in lieu of an actual calculation) slower than ship B and will eventually reach a point where it can drop in on top of ship B.

But wait, it's still at a different inclination from ship B, so how do we correct that? Remember when I mentioned ascending and descending nodes? Ship A can correct for the inclination at either of these points by either making a burn tangential to the surface of the planet towards the South pole at the descending node, or doing the same thing towards the North pole at the ascending node. So, once this burn is made, ship A will be on the same orbital plane as ship B, so now all ship A has to do is get into a lower orbit once ship B is below it using the same method I described earlier.

Now there are two ways you could avoid this. You could have battles take place in very high orbits so that the orbital velocity is always fairly low, though that would also make escape velocity really low, meaning that ships at this altitude would have to watch their speed to avoid going into an escape trajectory, and they would probably also have to use a pre-defined velocity as a sort of "zeroed" reference point for their own velocity. Either that, or you could just implement a technology that negates the force of gravity at certain altitudes to make space battle somewhat more feasible.

This isn't even going into the sort of weaponry you'd use. Depending on what types of weapons your ships are using, you'd have to take their orbital velocity into account as well. It wouldn't exactly do you much good if you shot a massive ball of tungsten carbide out of a railgun if it makes said projectile's orbital velocity zero. Without the centripetal force that orbital velocity provides, that ball is now plummeting towards the planet below, which could result in collateral damage, not to mention it will completely miss your target (unless you're going for an ad-hoc orbital bombardment). To compensate for this, make sure all of your guns use high-velocity rounds, projectiles reaching into the hundreds of kilometers per second range. That only really goes for kinetic types of guns, so you could use shit like particle projection cannons, lasers, quantum destabilization fields, or whatever the fuck else you can come up with to make up for it. Also missiles, missiles will work.

The convenient thing about the physics involving your weaponry is that you can avoid explaining it by just saying that it's all calculated by your targeting computers into a firing solution, or something like that.

tl;dr: If you want a truly realistic space battle, you're going to have to take orbital mechanics into account, and orbital mechanics is a bitch.