Since we are now running a Antistati server might be worth throwing up some tip on the forms.
Hit and run
Antistasi is a guerrilla warfare gamemode after all, and especially in the beginning it needs a slightly different approach to most things on ARMA. There are also some clever AI scripts that result in AI units coming to assist other AI if they're in combat. Once you get into a fight with one unit you can be pretty sure that more stuff is going to be on its way towards you, and in the early game in particular where you've got limited resources and weaponry, you're not going to win a drawn out firefight. Hit your target and run away quickly, don't hang around to see what else turns up. Which leads me to...
Be realistic with your objectives
It's not that hard to take over an outpost or a power plant. The hard part is keeping hold of it, because the AI will keep coming to try and take it back. You could stay there and defend it of course but then you can't make progress anywhere else so you're not achieving much.
Instead your initial mid/long term objective in a new game should be to do a bunch of missions from Petros and save up enough money and HR to afford a big enough AI garrison to defend something for you. Your HR is initially capped at 30 but that cap goes up to 60 fairly quickly. A decent garrison that has a good chance of holding a position without much help from you tends to be around 30 AI which is going to cost you about $7,000. Until you can afford this (preferably with some left over for reinforcements) there's little point trying to hold a position.
Understand how reputation and Factories work
Each town has a reputation figure for you, and a reputation figure for the enemy. The town will be "controlled" by whichever faction has the higher reputation there.
Each town gets its power from the nearest Factory to it. However, the town will only be powered if the side controlling the town and the side controlling their Factory are the same. If you control a town but don't control their Factory, the town will not have power and you will rapidly lose influence there (unless you keep doing missions there, but that's not sustainable) and pretty soon the enemy will control it again. The reverse is true as well, so if you hold a power plant any towns supplied by it that you don't control already will start shifting in your favour. The only real way to keep control of a town is to keep control of its nearest Factory.
Taking over a Factory which results in cutting off power to enemy controlled towns will cause your reputation with the allied faction to take a massive drop. Ideally do it when your reputation is low anyway, eg. just after you've spend most of it on some handy reinforcements.
Other quick tips
Put your base on a dead end road, or a road that is not the shortest route between two towns/bases etc, or even not on a road at all. You don't want a random enemy vehicle driving past and spotting you and machine-gunning Petros in the face.
For getting kit easily, raid outposts, take a pickup truck with you, and steal the crate of weapons and ammo that they always have. Run away and repeat another time when the enemy garrison is repopulated.
You'll mostly be using enemy kit so try to wear something distinctive and non enemy coloured so you don't shoot your friends.
Dogs can detect you even if you're incognito if you get too close (50m). Dogs are bad. Avoid dogs.
DO NOT FIRE MORTAR AT HQ!! enemy forces can triangulate the firing position and send a lot of pain your way.