Answer 1

depends on the language. i suggest
1) make sure you are decompiling legally
normally using the decompiled code can make companies really mad
just decompiling be careful though

2) see your tool
normally with the tools you are using, there should be some tutos for you to get an idea

3) see your language
windows programs can reveal a lot other programs not so, so, see what lang you are decompiling and master the tricks

4) get a good understanding of memory management