I have my system winterized every year, when I started it back up this year, I noticed a pretty significant leak coming from under the circled black circular thing on top of the system (see picture attached). This happened when I opened up the water valve in my house and then opened both valves outside. I haven't noticed this in the past so I think this is the first time. I am not sure where to begin. Each time I open up the right valve, which I believe is the one that allows water to go to the sprinklers, it leaks. I have posted a picture and not sure what other info you need. Any help you can provide would be greatly appreciated. Thanks!!
That part is probably an anti-siphon valve, required by code for sprinkler systems. These have a one way valve which prevents back flow of water in the sprinkler piping from flowing back into the house in case of a pressure reduction in the house. Turn off the water to the sprinkler system and open up the unit and examine it.
Simple cleaning of the seal may restore functionality, but you can get rebuild kits for these anti-siphon/back-flow valves. I think it has a replaceable seal which may deteriorate.