In windsurfing the basic tack is a 180 degree turn for beginners. In order to windsurf you need to be able to sail out, turn around and come back. There are a number or ways to turn around, but for now we will focus on the basic tack. Provided you can uphaul and sail in a straight line, you are ready to progress to tacking.
When sailing, the first thing you need to do is to turn upwind and not downwind. Turning upwind also prevents you from drifting too far downwind and saves you a long walk after your day of sailing. When you turn upwind, try and get the back of the sail to touch the back of your board, doing this will turn you upwind enough, putting you in position to initiate the tack.
At this point the wind should be blowing almost directly onto the head of your boom. Next, you'll to need bring your back foot up to the mast base, you can grab onto the uphaul rope or the mast itself if you need to.
As you walk around the board to the other side, keep the sail leaning towards the back of the board and try to move the sail over to the other side of the board while keeping your knees bent. Using your feet to push the board will also help. Once the sail is on the other side you should be facing the opposite direction with the mast base between your feet again.
Even if the sail dips or falls into the water, it's okay, you'll get better at it. The point is to first know how to turn and then keep practicing. Now all you need to do is pull the sail towards you so that it's back into sailing position. Look forward and sail away.