It turns out people have consumed rainwater for millions of years. So there's some precedent.
This is very well covered online, so grab a search engine and get reading. Here's a summary of concerns.
In the US, it may be illegal (look up "water rights") but for individual home use, that is usually ignored.
You'll get a lot of water from your roof really fast. It's pretty amazing.
The biggest problem is the dirt on your roof. The rainwater coming down is great, but once it hits your roof, it's dirty. You can redirect the first bit of rain, and then collect once the roof is clean(ish).
The roof material matters. Some roofs will leach small amounts of toxins in to the water that you probably don't want to drink. Metal is probably the best choice for a roofing material.
For most homes, only a small portion of the water goes to drinking. Much of it goes to laundry and flushing toilets and irrigation. These systems don't mind slightly dirty water (including greywater). Use your collected rainwater there first. You can haul it by bucket or plumb something special.
A problem specific to collecting rainwater for irrigation is that it always comes at the wrong time. You would have to save it from the wet season to use in the dry season, which implies a huge cistern. Being clever helps. For example, a cistern can go under a deck.
Any water storage should be protected against mosquitoes. You can put fine mesh screens over openings, or (my favorite) put goldfish in the tanks to eat the larvae.
Cleaning dirty water to be drinkable is harder. If you build a complex filtration system, you'll probably negate all the financial and ecological benefits of saving that small amount of water you were drinking. My favorite approach is the slow sand filter - it's simple, cheap, low energy, but it takes a lot of work. read here: http://www.homestead.org/TedPraast/SandFilter/Filter.htm
Oh, wow. I'm sorry, but your roof is probably bad. If you can get the money you paid four years ago back (doubtful), I would, but you probably need to get that entire mess torn off all the way down to the decking, and probably quite a bit of the decking near the edges of the roof too. You can tell because the shingles look "lumpy" and have a rolling look to them -- like someone's melted them, especially near the edges -- and the valley is curved instead of sharp. I can also see the lines running across where different runs of roof sheathing panels meet (every 4' or so); that it's sagged in between these lines is a sign of sheathing damage.
Your roof isn't the shingles, so it doesn't matter that the shingles are only four years old. Your roof is actually the tar paper and ice and water shield that is underneath underneath the shingles. Multiple layers of shingles are bad; it means that they didn't replace the actual stuff that keeps the water out of your house, they just replaced the stuff that you see that goes over top.
If you go up into your attic when it's been raining quite a bit, see how much more of the underside of your roof sheathing is damp. If there's a lot, you need to get it torn off and replaced as soon as possible. If there isn't much, you have a few years to save up for it, but it looks bad now from the top. Make sure when they re-do it that they tear everything down to bare wood, remove all the nails and staples and junk, use Ice & Water Shield along the roof edges and in the valleys, and then put new 30 year tar paper and shingles over it. The tar paper and ice&water shield is what actually keeps the water out of your house. The shingles only protect the tar paper from abuse.
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I was going to suggest rainhandler as well - because I'm a civil engineer and I think it's a neat idea - but not because I've ever actually seen it used (it is not a common product). However, I would not install on my own home if heavy snows or ice were a big concern.
I believe what you're looking for is a simple drip edge, similar to your picture. Do a quick search for "drip edge" or "drip flashing" and you'll find all kinds of products (many of which are likely available at your local home improvement store). Drip edges are often installed with a gutter system, but can be installed on their own as well. A drip edge is just a strip of metal or plastic that moves water droplets a short distance away from a building and then lets them fall straight down.
As you've shown in your picture, a drip edge will usually have a 45 degree bend at the end. This bend is what makes the water droplets fall down, as it is very difficult for them to travel "back up" the backside of the bend. If your flashing were flat on the bottom I.e. a 90 degree bend, then it would not work well because water droplets could wick back to the building by traveling along the flat bottom.
One problem that I see with your picture is the placement of the flashing - there's no overlap from the metal roof to the drip edge. Drip edges are more commonly installed at the top edge (top of the fascia), often under shingles. The idea is that you want water droplets to roll off the shingles or metal, onto the drip edge, and then down to the ground. In your picture, water droplets would roll off the metal roof, down the fascia, and then (hopefully) down the drip edge. I say hopefully because most likely you'll end up with some sort of gap (however small) between your drip edge and the fascia. This little gap is where mold likes to grow and water likes to sit to rot away your fascia. Installing your drip edge at the bottom is fine, but I would then choose a product that covers the fascia too, and then goes up under the metal roof (the drip edge needs to somehow start underneath the metal roof). You want a continuous path of overlapped materials for water droplets to flow down - like how shingles always overlap the next one below.