Drains can be a lot of fun. If you already go spelunking, you know a bit about being underground. The biggest difference about drains is that they are manmade and a lot more uniform than caves. A few safety tips:
1.No Drains when it rains. They can very quickly fill up with water and potentially suffocate you. Always check the weather report before entering one.
2. Don't open random manholes from the inside to get out unless you know EXACTLY where it comes out. A lot of manholes are in the middle of the road and you can be seriously injured if you pop out when a car is coming.
3. Bring extra everything: lights, drinking water, batteries, food, etc..
4. If you get lost, follow the flow of water. It should lead to an outflow in a lake or river.
I'm not familiar with the laws in your area, but its usually illegal to go in them. But if you make sure you that no one sees you enter or exit the drain, once you are inside you should be safe. Cops don't randomly wander storm drains.
A good reference about draining with a lot more detail is
http://sleepycity....proach-to-draining Have fun and don't be too intimidated. Just go for it.