C# and Java are very very similar.. they aren't identical, but the syntax is basically the same, and a lot of the methods are similar (like ToString() in C# where it is toString() in Java)
C# is really easy to program plugins in as long as you have some decent programming skills..
just be aware of the rules, the Code of Ethics, and that Decal does not make a good first programming project. It is mainly for people who have a solid programming base, and want to extend their abilities.
The basic rules are at :
http://jvarga.berkeley.edu/decal/rules.htm
while most everything else can be found at :
http://www.decaldev.com