After you drain it, muck out the leaves and any thick gunk. A little dirt is ok, that will contain beneficial bacteria which will help with water quality. Once you refill it, expect an algae bloom in a few weeks, that is normal. Add some plants in pots in the spring. Does your pond have a shelf or shallow area? That is a good location to set the plants. Where abouts do you live? Your climate will affect what will grow in your area. Is the pond in the shade or in the sun? That will also affect the plants you grow, and how much algae you get.
As for keeping the pond clean, every week clean out leaves that fall in. Is there a pump to circulate water? Is it in the bottom of the pond, or elsewhere? If you need to buy one, what you get depends on what you want the water to do, as in just circulate, have a little waterfall, etc. and how many gallons your pond contains.
A water test kit is not essential if you don't plan on having critters. Stagnant water gets smelly in time, so a pump to move water is important.