One thing that I've noticed when you hand a student a triangle and a beater and ask them to play it, is that the first thing that comes out of the instrument, sounds like a fire bell. Playing a triangle correctly is not easy. It can be one of the most abused instruments in the percussion section. I've found out over the years, that you don't just take a triangle and hit it. You have to pick the right instrument for what you are playing, select the right beater(s), and have a good clip that lets the instrument vibrate.
There are many brands of quality triangles on the market. Each has its own sound concept, and which concept you choose is a combination of personal taste and musical requirements. I own triangles made by Alan Abel, Ron Snider, and Grover Pro Percussion. My Grovers are my go-to triangles for most applications. I have two, large and small, made from some sort of steel alloy. I have three that are made from brass. I also have two different Abel triangles, an old Ludwig triangle, and a large brass triangle made by Ron Snider of the Dallas Symphony. The Snider triangles are no longer being made. Each triangle has a completely different sound, and I pick the sound I am looking for based on the requirements of the music being played.
One of the most important things about playing a triangle is what are you using to suspend it. I've seen triangle clips in schools using shoe laces to hang the triangle. They wonder why the instrument doesn't ring. It sounds like the famous "come and get it" triangle sound from the old western movies. Personally, I like the wood triangle clips made by Grover. The wood doesn't transfer the sound of the music stand to the instrument if you are playing it with two beaters, like you would in Brahms Symphony #4. They also have two suspending cords that are made out of a knobby plastic material that minimizes contact with the instrument. One is a safety in case the other one breaks. You can buy replacement cords from Grover. Always check the condition of the suspending cords before you mount the triangle. Many players like to use monofilament fishing line to suspend the triangle. It works great, but make sure you have at least one safety line. Two are better. Old school purists like to use gut to suspend the triangle. Just don't use a shoe lace, a piece of twine, a rope (yes, I've seen that too), or anything that will impede the vibration of the instrument.
Quality triangle beaters are a necessity. I use the Stoessel beaters. You want a beater that doesn't produce a "tickey" sound. The Stoessels are the best that I have found. They are also expensive. However, you get what you pay for. I hear the Grover beaters are good too, but I have never tried them. I may order a set in the near future and give you an update.
So how do you play a triangle? There are basically two ways. One is you hand hold it and strike it with a beater. The other way is you suspend it from a music stand with two clips, and you play it with two beaters. When playing a triangle that is hand held, you strike the bottom bar at a 45 degree angle with the opening to your left if you are right handed. South paws do the opposite. You play a roll by moving the beater in the corner in a rapid up and down motion. For two handed playing, you suspend the triangle on a music stand with the bottom bar of the triangle on the top and the angle hanging down. You then play with two beaters like you would any other instrument. Rolls are done with both beaters. Would you ever use a drum stick on a triangle? Unless the composer specifies it, the answer is no...with one exception. If you are ever playing triangle in a Zydeco band, get the biggest triangle you can find, hold it with one hand rather than with a clip, and strike it with a drum stick. You want a dead metal thud sound for that kind of music. Go grab a couple of triangles and experiment. You'll fall in love with the instrument.