If you own a lawn or plant sprinkler system, at some point you will have a leak. It is inevitable. Just accept it. Leaks can come at the actual head tip, on the sprinkler case, between the sprinkler and the shutoff valve, or before your shutoff valve. Most leaks occur around the head due to damage to the head or sometimes due to freezing damage.
For most leaks you will want a plumber if you are not knowledgeable about plumbing repair, but some leaks can be easily repaired as a part of your regular sprinkler maintenance. If your sprinkler tip is leaking, you will probably be able to simply tighten the tip. If you do so, remember to rotate the whole sprinkler to realign the spray pattern. If your sprinkler is leaking from under the sprinkler, check to see if the leak is coming from where your sprinkler is screwed onto the riser that comes from your pipe. If so, you may simply need to tighten the sprinkler down or unscrew it, replace the Teflon tape around the riser threads, and return the sprinkler to the riser.
If your sprinkler case is leaking where the popup screws into it, remember that is generally not repairable. Make an attempt to tighten the connection, but you will probably have to replace the whole sprinkler head. The reason for that is once the o-ring has been damaged, dried out, over-compressed, or covered in calcification, it is not able to correctly seal again. It is possible to use some PVC glue to stop the leak for a short amount of time, but eventually that will no longer hold.