How to Connect Plastic Pipe to Metal Pipe
Plastic to metal connections are made using threaded connections. A plastic male thread is used to connect to metal female threads.
Do not use a plastic female threaded connector with a metal male connector! The joint will almost always leak or break.
The standard IPS type threads used on pipe and fittings are not uniform in diameter. For example, if you look closely at a male pipe thread you will notice that the diameter at the first thread on the end of the fitting has a smaller diameter than the last thread. Likewise with a female thread the first thread at the end has a larger diameter than the last thread. There is a good reason for this. When you are screwing the threads together it actually does get tighter! As the threads are screwed together it starts to compress the male fitting while stretching the female fitting, resulting in a very tight joint that doesn't leak. This is why it gets so much harder to turn the pipe as you continue to tighten the joint. However this creates a problem if you are joining two different materials together. If one is softer than the other, then the softer one will do all of the compressing or stretching. Compressing is generally not a big problem, but stretching too much can be bad news for the strength of the material. To adjust for this effect, female threads on fittings and valves are often reinforced by making them thicker. Look at a typical metal female fitting like a coupling and you will notice the reinforced end. For plastic valves they not only use thicker plastic, they sometimes will place a metal ring around the female end, or put fiberglass reinforcement in the plastic.
Plastics like PVC create more of a problem since plastic easily stretches compared to almost all metals. Because of this, you should (almost) never use a female PVC fitting with a male metal fitting. Since the male metal thread is harder, the male threads don't compress and all the "give" comes from the female PVC. The result is the female PVC is stretched beyond it's strength limits by the hard metal, resulting in tiny stress cracks and leaks. The exception I hinted at is that there are some specialty plastic fittings available that have a combination of heavier plastic and metal reinforcement rings to give them sufficient strength to withstand stress cracking. Typically these fittings are only used on large agricultural, park, and golf course irrigation systems.
There is also another type of threads called "acme threads" that do have uniformly sized threads. These are the type of threads used on items that need to be easily disassembled, like jar lids.
Sealing Tapes and Pipe Dopes
When making threaded joints you need to use a sealer/lubricant. This is a product that helps seal the joint and also serves as a lubricant to make it easier to screw the male end into the female. The standard products for this are either Teflon pipe dope, Teflon tape, or PTFE tape. For sprinkler systems Teflon or PTFE tape is preferred. If pipe dope gets into the pipes the water will carry it to the sprinklers and it will gum them up (using pipe dope also voids the warranty on most sprinklers.) Some pipe dopes made for metal pipe are not compatible with plastic. They will harm the plastic resulting in future failure. Stick to Teflon or PTFE materials that are labeled for use on plastic.
If using pipe dope spread it on the male threads. Try not to get any on the last thread on the end, so it doesn't get squeezed into the inside of the pipe. Put a good thick coat on starting with the second thread from the end. Most pros use Teflon pipe dope because it is faster and takes less thinking than Teflon tape to install. (Teflon tape must be put on in the correct direction or it comes off.) But they pay the price with ruined sprinkler heads they have to later replace.
Some Teflon tape, sold typically at discount suppliers, is very, very thin. If you can hold the tape up to a light and see the light through it you have some very thin tape and you will need to use a lot more of it. Use about 9-10 wraps around the male threads when using the cheap thin tape. I find it is actually cheaper to buy the more expensive thicker tape.
For good-quality tape it takes 3-4 wraps around the male threads to create a good seal. Don't put tape on the first thread and you will find it easier to get the joints started. Pull the tape tight around the threads as you put it on, but don't stretch it too much. You should be able to see the shape of the threads through the tape. Wrap the tape around the pipe threads in the same direction as the female threads will screw on. (Clockwise if you are looking at the end of the male threads.) If you go the other direction the tape will tend to come off as you try to thread the ends together.
I prefer to use the pink colored "extra-heavy", "full density" PTFE tape. I almost never use pipe dope.