Spend the beginning waves buying guns and ammo and just running and gunning. Then a few barricades and turrets. If you go the first 6 waves without spending perks or upgrade points you can get most of the upgrade achievements out of the way so you can spend them on what you want. I go over this in my video at 19:20. If it's too hard you can by some upgrades and do it in later waves.
Achievement Walkthrough - How To Survive 30+ Waves: