The first strategy for heating water is very simple. You need a black metal container with a lid to put the water in, a large clear plastic bag that will cover the container, one that has lots of extra room that you can inflate (an oven bag for baking turkey works well), and finally a twist tie and a large shiny metal reflector of some sort (this is where you get to be creative).
Put the water in the black container and put the container in the bag. Puff up the bag with air and twist-tie it closed securely (to keep the heat in). Put the bag and container in the center of the reflective disc and point it directly towards the sun. Leave the whole contraption in the sun a few hours to heat the water.
The next strategy is to buy or make your own solar oven. This has the added benefit of not only boiling some water for your tea, but cooking a small chicken for you at the same time. When built with care, a solar oven is more durable, hotter and easier to use than the previous method. The main components are an insulated black box with a lid, some sort of glazing (the most popular being glass), reflectors (such as mirrors, shined aluminum or windshield shades) and black pots that you can find at army surplus stores. The black pots go inside the insulated black box, under the glass, with the reflectors on the outside bouncing extra light towards the pots. According to Solarcooking.org, a solar oven with just one reflector can reach 300 degrees F. Add additional reflectors, and that oven will get pretty hot. Always remember to use oven mitts when taking your container out of the bag or oven.
If you decide to build a solar oven, don't try to reinvent the wheel. Look at many different plans to determine what materials and design types will be best for you.