Okay first of all insure what was in the drums originally. If it was a petroleum product don't bother with them. Next figure out if you are going to catch water off a roof which has a downspout system, or if you need a free standing system. It's easy enough to make a frame with light PVC and poly tarp, but you have to be ready to deploy it, stake it down, and then break it down after it rains. I put in a cistern this summer and it has worked well so far.
to be safe try to find food grade storage barrels. if you want to know where to find them go to your local Spanish grocery store or other low class grocery. chances are they'll be selling the barrels the food came in. they maybe a little smaller then 55 gallon but they are huge food grade so you will have no problem using them for water, and they should only cost about 20 bucks, try to haggle and get them for cheaper. the places probably want to get rid of them.one buy my dads job always has them, also this other Spanish supermarket we use to go to.
if your just gonna take the rain water off your home by an adapter for your down spout, their flexible so it will be easy to work it into the barrel. chances are in one hard rain you'll more then fill up your barrel. it will be grey water so unless you filter it really good i wouldn't suggest drinking it. but it will save you on things like watering your garden and other things you might need water for. save you money now and save your good drinking water later. ie after the zombies rise.
if you want to use it for drinking you'd be better off making a funnels for it like DR said out of PVC and tarp. good luck