Back in the day any corporation or alliance could declare war on anyone else. But those game mechanics have changed, and now a corporation is only war eligible if they own a structure such as a citadel, control tower or even a planetary customs office. Mobile Depots and Mobile Tractor Units don't count as war eligible structures.
There is however a clever trick where you can create a shell corporation with an alternate character (even an alpha character) and have it own your structures, and then delegate access to your main corporation or alliance, although it doesn't really work with control towers. But this way if someone declares war on your shell corporation, your main corporation or alliance will be unaffected, unless you decide to join the war to protect your assets.
But what to do if you end up in a highsec war? Let's talk about it.
If your corporation or alliance is war eligible, it makes sense to create some alternate characters (alts) for highsec activities. You can create 3 characters per account. Basically during a war you shouldn't be active in highsec with your war eligible characters. This means no mining, no mission running and no other PVE activities that require you to undock.
However the main thing you don't want to do while at war in highsec is hauling. Consider either using a freight service such as Red-Frog or PushX, or then create an alternate character for hauling. You can start with a T1 hauler such as an Iteron Mark V or a Wreathe, if you prefer speed, and work your way up to deep space transports, blockade runners, Orcas and freighters.
I would also recommend training trading and contracting skills for your hauler alt. This way you can use them as a trader in a trade hub to buy and sell items while you are not able to do it with your main character during a war.
Another alt you might want to create is a scouting alt. Ideally it should have a covert ops capable ship, such as a covert ops frigate or Astero. But a regular T1 frigate with a Prototype Cloak is OK too. You can use this ship to scout highsec systems for war targets. That said, it can be used in regular scouting activities too. This can be the same character as your hauler alt, training the minimum requirements for a scout won't take long. It's even better if you play with two accounts at the same time, then you can scout yourself much more effectively.
NOTE: DO NOT have your hauling or scouting alts in war eligible corporations.
When you are at war in highsec, some mercenaries and highsec pirates mainly camp trade routes and trade hubs. Systems like Jita, Amarr and Uedama, as well as surrounding systems, will be exceptionally dangerous during a war. But other groups will actually hunt down the members of your group in highsec. That is part of the fun of wardecs. They can use locator agents to find the location of characters anywhere in New Eden (except wormholes) and send their spies with covert ops capable ships to locate your specific location within a star system.
However many of these wardeccers are usually not eager to enter dangerous systems in areas such as lowsec, nullsec, wormhole space or Pochven. If your corporation or alliance has a base in one of those areas (which is a good idea), you should consider relocating there for the duration of the war. You can use your hauler alts to transport items to and from the nearest highsec station, or perhaps into lowsec or wormhole systems if the route is safe.
It is also recommended to gather intel on the group that is declaring war on your corporation or alliance. This can be done by looking them up on the Zkillboard.com website. Just go to Zkillboard and type in the name of the corporation or alliance that has declared war on you. There you can find their killmails, which will give you intel on the ships they fly, what they tend to target as well as when and where they are most often active.
With this data it becomes easier to more safely coordinate your movements in highsec for war eligible characters. That said, if not for PVP purposes, I would avoid moving in highsec with anything other than a shuttle or perhaps a covert ops ship.
There's a few reasons why people declare wars. For example some other group may hire a highsec mercenary group to declare war on their enemies or to destroy a station that they can't do themselves. But one of the more common reasons why this happens is because the wardeccers want easy killmails on their killboard page.
So try not to lose any ships in highsec while you're at war, as this may prolong the war declaration. If you're going to explode, do it outside highsec and not to wardeccers, or in something dirt cheap, like a shuttle. But shiny kills will put a bigger target on your back, which will negatively affect the whole corporation or alliance. Therefore, be mindful of what and where you are flying during a war.
That's all for now, good luck in your wardec!