- Learn what the various units, abilities and upgrades do
Dawn of war is very "rock paper scissors"-esque with most units being strong in a specific situation (e.g. against vehicles) and very weak in other situations (e.g. against mele). In order to win you really need to be able to understand both what the enemy units weaknesses are (e.g. what units should I use to destroy that tank) and also what your units weaknesses are (e.g. what enemy units can destroy my tank)
While you are playing hover over each of the unit build icons / upgrade icons / ability icons when you get a chance to see what it says. Try building at least 1 of each unit / upgrade at least once and try out each ability a few times on different targets to see what they do.
Also play the other races as well - its just as important to know what the enemy units do.
Generally speaking you shouldn't send units to their deaths - keep them alive whenever possible by backing away from battles you know you can't win and retreating units that are really in trouble (hotkey X). The disadvantage of retreating units is that they will be out of the action for the time it takes for them to run back to your HQ, however while retreating often allows you to save units (and therefore resources) that otherwise would have died.
Beware of retreating too early - if your forces spend their entire time running backwards and forwards between the battle and your HQ then thats also a waste of resources.
Also beware of retreating too late - even if your unit survives it costs money to reinforce. For example if an Assault marine squad gets overrun by mele units it may be a good idea to retreat them immediately even before they take that much damage - keeping them in the field will at best keep that mele unit occupied for the amount of time it takes for them to be killed so why take the casualties?
- Use cover where possible, except when you shouldn't...
Cover plays a large role in DOW2 and often makes the difference between winning and losing a skirmish. Generally speaking you should use it whenever possible, however its worth noting that some weapons (e.g. flame throwers) do additional damage to units in cover. Try to use cover effectively but also keep a note of what weapons and abilities you want to move your units out of cover for.