The AI in Dawn of War is pretty shocking at the best of times and frequently gets stuck at the first tech level spamming small weak units in an attempt to complete a "tech 1 rush". Typically you will find if you can hold off the initial wave the rest of the game will be a piece of cake seeing as you will have vehicles and they will still only have infantry.
My main point of advice would be to use lots of little squads instead of upgrading one squad (i.e. I would rather have 3 different marine squads of 4 men as apposed to 1 big squad of 10). This means you can engage multiple targets at once and if someone gets drawn into melee when you didnt really want them to you have another couple of squads to help clear the trouble).
As for how to hold off this initial wave it depends on which army you are playing as and which you are fighting. For any army I would definately say make sure you build a command unit as your first or second attacking unit seeing as the extra HP will definately give you a bit more thinking time (this is especially true of necrons seeing as the necron lord is the best unit you have at your disposal) ... from here its a matter of who you are fighting and knowing how they will rush you.
Marines / Choas - Will usually play a solid game with decent AI - harras you a little at the start with marine squads but continue to push through tech levels so you will need to keep up with them technologywise. With choas be careful of cultist squads because they have a tendancy to engage multiple squads in close combat at once and leave them prone to fire from another unit.
Elder - they will typically throw guardian squad after guardian squad at you and then throw the occassional farseer whenever they have enough spare rec (strategy = get heavy bolters out asap - or whatever your armies equivielent is).
Orks - Similiar to elder, will begin by throwing squad after squad at you but with the added danger of a big mek. This guy can be a pain in the ass because of his teleport ability which will get him to you sooner. (Strategy = you will definately need a commant unit to deal with an ork rush). If orks manage to get to get to tech 2 they are likely to start throwing wartraks at you, counter these with vehicles because their rockets will constantly knock your regular troops over.
Imperial Guard - Again similiar to elder, will throw lots of little units at you as well as the command squad which can be a pain because its quickly regenerates powerful units. Heavy bolters (or similiar) are the way forward again.
Necrons - On big maps these guys will hardly have the chance to rush you because they are so slow, but you will need to be careful of the necron lord seeing as his teleport power recharges very quick and he can get to your base super fast because of it. Engage this guy with a command unit and have everything else you have shot at him. Probably best to keep his body surrounded as well because when they rebuild him it will revive in the same place it was last killed.
Tau - Watch out for the krut rush, these can be nasty especially if they have feral leap. Lots of seperate units will help with this one so that all squads are not simultaneously having to engage one squad.
I didnt ever really play that much soulstorm so cant realy say more about sisters / dark elder.