hey there fishyz,
you first want to look at some oxigen plants to keep your pond clear. The problem is because it sound like a small pond and it is in direct sunlight the water may get to hot for most plants to thrive. Is the pond going to cool down in winter, because if it does you will have another problem and that is that the plants don't like to get frozen. So having this in our minds I will recommend you the following:
1 small nymphea spec. (waterlilly)
to cover 1/3 of the water with the leaves to prevent extreme heating from the sun
1 (or more if you have the room) Iris pseudacorus
wich you can plant at the side in shallow water, it will adsorb a lot of nitrates and stuff like that
then some Egeria densa unter the water
they will look nice unter water and make shure there is enough oxigen in the water, they will also prevent the grow of unwanted alge. The also provide cover for the guppy's
If it is a really small outdoor pond that will get completly frozen in the winter you might want to consider something like rallysman suggested. Some big peaces of bogwood with lot's of java fern may work. The best part is you can take them indoors in the winter and just place them in a temp. holding tank for the guppy's. Same gows for the Iris if you plant it in a holding crate for plants.
Your likely to get alge in the pond because of the heating menthioned earlyer. The best thing to do is just make shure you provide some shade during the day (best is to have it in the shade from 11:00 till about 14:00, because the sun is the strongest during that time of the day. Or at least here, maybe someone else know when the sun is the strongest during the day at your place).
If you give some more details about the pond, like size, with or without filtration, etc. I can be of more accurate help.
Greats Loubard