Initial assumptions: your spaceships' cababilities don't include things like teleporting the entire ship, ignoring G-forces, effortless FTL travel. Ships are much smaller than, say, a planet.
Tl;dr: You'll mostly be meeting your opponents around assets like planets and space stations, which affect the way battles are fought. Information warfare is central and requires physical access to ship systems.
Your objective determines the battlefield
Space is big, mostly empty, and spaceships moving at relativistic speeds are hard to see coming and/or intercept in time. Battles would usually take place near planets, space stations, dyson swarms and other such points of interest, even if your target is specifically a spaceship.
Other large objects in the vicinity (that you can't just vaporize because they're either very valuable or massive enough to withstand your fire for a while) will affect tactics.
Open space combat is deadly
Whether you're zooming past your target at near light speed and dropping projectiles in their path or exchanging missile barrages from two light seconds away, the most likely outcome for roughly equal opponents is mutual destruction.
If you intend to accomplish anything worthwhile in space combat, you need to use the "battlefield" to your advantage.
Scenario: Taking a planet
A ship/fleet sent to conquer a planet or a large space station would probably need to be designed with that purpose in mind. They'd start firing at the target's stationary weapons and general vicinity far in advance of their arrival, hopefully hitting enough to be able to approach without being torn apart by counter fire.
Defenders near the opposite side of the planet will be mostly unharmed, so the attacking force would be wise to take advantage of this cover as well. Maneuvering up and down the horizon and attempting to snipe one another with railguns is still an option, but guided missiles going around the planet don't require you to expose yourself. These, on the other hand, are slow enough to be targeted by all sorts of point defenses, the most powerful of which have been placed planetside or in low to medium orbit to avoid the initial barrage.
Surviving forces on the planet and in orbit will easily outnumber ship crews, and with point defenses struggling to keep up with saturation bombardments, boarding parties have a decent chance of getting to their destination.
Intelligence is key
Electronic warfare can turn a battle in an instant. Courses and tactics need to be communicated. Point defenses and targeting systems need split-second coordination. Turning one enemy railgun against their allies is a guaranteed kill. Broadcasting bogus telemetry can crush a fleet.
But in an equal contest of wits and processing power, the defender always wins. You need physical access to their systems in order to hack them. Blowing up one of their 25 redundant processor banks won't accomplish much. Allowing a team of soldiers and technicians to plug into it will get the job done. And who knows what kinds of useful info they might find in the process?