C# .net / mono cross-platform differences?

I've been searching for a while now trying to find an answer for this, basically if I create a visual studio app using .net c# and I want to port it to mac / linux,

1. Will there be and code I'll have to move around/change when switching to mono?

2. What things should I avoid doing while developing to make it easier to port later?

3. Is system.windows.forms usable in mono? Should I just use GTK#? Does it matter?