I want a cheap solution for water heating, maybe even something I can build myself, please give me some ideas
Unfortunately nothing in life is incredibly easy, and solar is no different.  I would NOT suggest attempting to install it by yourself or even with the help of some untrained friends.  Most average people wouldn't attempt to install their own electrical, or even their own water heater, but for whatever reason, some people believe that solar is easy.  It's not.  If you can't install your own water heater, you shouldn't even think about installing solar yourself.

What you can do, however, is analyze whether or not you are a good candidate for solar in the first place.  You might find that you are a great candidate for solar, or you might be able to mark it off the list, narrowing down your options, making your decision eaier.

Question #1:  Since you are in the Southern hemisphere, you want it on the North side of your roof.  How much available roof space do you have on the North side?  Solar hot water panels, for typical homes, will require an area between 9'x9' and 18'x9' in size (horizontal x vertical.)  (Note: If there is no room on the Northern roof, but you can install them on the ground near your house, then that is also acceptable.)

Question #2:  Are there any trees or other objects that will shade the Northern side of your roof (or the ground location)?  If so, would it be reasonable to remove the objects causing the shade?  Shade between 9:00 to 3:30 should be avoided, and shade between 10:00 and 2:30 is unacceptable.

Question #3: Is your storage acceptable for solar?  You should have roughly 10 gal + 20 gal/person in storage.  You should have an electric/gas heating element within the last 40 gallons of the hot water outlet... and plenty of storage near the cold inlet that is heated exclusively by solar.  The element near the hot out is there to heat the water in case there is no solar input, and the storage space heated exclusively by solar is required so that the solar can have a chance to heat the water without having to compete with the electrical/gas element.  For this reason, a two-tank setup is ideal.  In the diagram below, the cold water enters the tank on the left, which should not be plugged in or connected to gas, instead being heated exclusively by solar, which feeds the tank on the right, which should be heated by gas/electric.  With this setup, solar should be able to provide ~85% of your hot water.



Question #4: How many "sun-hours" do you receive in your location?  (This is not equal to simply how many hours of the day that the Sun is out, but is a measure of how many watts per square meter fall at your location.  It is a product of intensity/angle, duration, and cloud cover.  Any measurement that you might find from a location within 100-150 km is sufficiently close.)  Low exposure to the Sun can be compensated for by installing an additional panel, but if your area gets under 2.8 sunhours/day, I don't know that I'd recommend solar.

If any of the above concerns are applicable and not rectifiable, then you can cross solar hot water off the list.  Unfortunately, not everyone is a good candidate for solar, and solar HW systems that are designed poorly are a waste of money, but well-designed systems installed on homes that meet these minimum requirements work exceptionally well and will produce hot water at a fraction of 15 cents/KWh electric.. or even natural gas.  Though certain components will need replacing, sturdy flat-plate collectors should last 50-150 years (in a drainback system) and withstand up to baseball-sized hail.

Though I cannot install it for you, very detailed information is available on my web site's page on how solar hot water works.