Posted: Hi! Good choice in learning to hoop- sure you'll have fun with it! Common advice is that your first hoop should be up to about the height of your navel, but you might find this is a bit big, especially if you're slim. If you know anyone else who hoops, or have access to a choice, really, having a go at a load of different ones until you find the size you like is the way to go. You'll probably find you prefer a smaller size (or at least a lighter weight) after a bit.
As for making your own hoop, most are made of some kind of plumber's rigid plastic piping, often with a diameter of 20mm or 25mm. The name and exactly what to ask for depends on where you're from. There's some good resources on Hoop City. The main problem is getting small enough lengths of piping, as you only need 3-4 meters for one hoop, but it's sold as 25 or even 50 meters. Also connectors that actually *match* your pipe can sometimes be fun to find.
You can buy matching parts in the shop here, but you can get it at DIY shops- there's so many hoopers around here, I got asked if I was making hula hoops with it last time I bought some from a trade store
Posted:Hamamelis pretty much covered it. Although I will add that the psi rating on the tubing may want to be put into consideration. The higher the number the heavier the hoop will be. One of mine is 160-fairly standard for a hoop- and another is maybe 120? The lighter one is more for dancing than exercising as it creates less resistance and is a bit harder to control if just starting.