This is the way I play it - it may work out differently for you, but I always get to at least lvl 24 using this method.
First, get a mochigames account - the enfield at the start really helps.
Okay, until lvl 5 it's pretty simple - just use the rifle and defend the first room.
As lvl 5 comes to a close, unlock the street (50) and reception (150), and get the shotgun (125). This is a really useful weapon until about lvl 16 - at close range it kills normal zombies in a single hit, and has no limit on the number it can hit - if you bunch up an entire wave of normal zombies (blue and green) and shoot them at very close range they will all die on the first shot or maybe the second or 3rd.
Next, once you have $450, upgrade your walls to the brick wall (150 + 300), then just wander around killing them with the shotgun until you have $4500 (should be around lvl 17 - a little skill may be needed to get there). Then, quickly unlock the mess hall (500) and captains office (500) and get the assault rifle found in there (3500). This is fully automatic version of the Scar - a little less powerful than the semi auto assault rifle, but it mows them down amazingly. Keep killing zombies. I find that if you stand by the barricade in the reception, all the zombies from the left side of the map come in a straight line - easy to mow down. Reload whenever you have breathing space.
The next most important thing is to save up until you have $6500, to get the last two barricades (1500 + 5000). The final one damages zombies that attack it - so make an acive effort to repair them regularly. Upgrade them at sensible times - each upgrade repairs all your barricades - it may well be useful to wait till they'r all about to break or broken before upgrading.
This should have occurred by about lvl 22 - now you have the choice to get a machine gun or just lots of sentry guns. I chose the sentry guns. Run down and unlock the vehicle bay (150) and the workshop (800), then unlock the sentry guns. When you have $4000, place a $4000 one at the doorway between the reception and the street. Stay in the reception from now on. Keep killing - by the end of level 23 you should have another $4000, so get another sentry gun and place it nearby the other one. At level 24, the dead skeletons start attacking you (they pop up from the dead zombies), so watch out for that. Also, a few demon-like things will attack - they take A LOT of bullets, and throw something like fireballs at you for hefty damage. I rarely get much past this point - it may be worth buying a machine gun and seeing if that helps at all...
Hopefully this will give you a good bit of help though!