Spilt the water on the keyboard

Spilt a bit of water on keyboard. I noticed that "minus" key button does not work at all. Before, it was stuck and then stopped working. Should I open the case and try to clean it, or do something with it? I have no money to replace it with new one.

Water might have shorted out the switch to the escape key, at least your macbook is still working. If your warranty expired and your tech savvy you might want to try opening it up and cleaning up any remaining water. iFixit.com has great write-ups for disassembly and reassembly.

My warranty isn't expired. This button works, but it takes me great pains to do it work by pressing enormous times and still it is stuck.

Take it in and explain that the key is not working and pray they don't find out it got water on the board. If you attempt to repair it yourself you will probably void your warranty.

Water spillage isn't covered by your warranty. Check your home insurance instead.