Will Minecraft warn me before charging me money?

Minecraft will not automatically bill you for connecting to ANY server.

If you have paid for a Minecraft account, there's nothing else Mojang will ask you to pay for. Online play is completely free. The only thing you would ever pay for is if you hosted your own server, and that isn't through Mojang.

Like everyone else said, you can't be charged by going on servers. Server owners may ask you to donate, but this isn't linked with Mojang and obviously you don't have to. Once you buy Minecraft, you don't have to pay again.

Once you buy minecraft its free if you go onto a server its free etc

First of all it costs nothing to log onto Minecraft servers- and Mojang (the people who own Minecraft) don't run the servers, they instead rent them out to people who do want to run one.
These people can't charge you money- they don't have your credit card details and Mojangs servers won't transfer money.

What some server owners will do is grant special privileges to players on their server if they 'donate money', but this is done via a separate website which they'll have written in the chat or on signs, and the server owner usually made this website themselves and is in NO Way connected to Mojang.
There's no possible way they can force you to give money either.

Also, you don't have to pay for ANYTHING on Minecraft- once you buy the game that's it, you don't have to pay a monthly fee, you don't have to pay for updates, nothing.

^-^ Hope this helps! ^-^

Some servers request payment to defray the cost-of-operation. These servers work by white-lists. Basically they won't give you permission to join unless you have already paid.

Not all servers want payment, there are plenty of free ones out there.