For pipe welding there are a few different things you'll need to know to help you. A lot of construction work requires measurements, so you'll need to be good at addition and subtraction when you're working to figure out how long to make the pipes, and where they need to be located. You also need to understand geometry to understand and measure the different sizes of the holes of the pipes. Most pipes are circular, so you'll need to understand diameters (how wide the circle is), and also the thickness of the pipe.
So make sure you study basic algebra and geometry. Also, you can start learning how to read a measuring tape and also how to read construction drawings.