On spells? Use Trigger editor in Wc3 editor. For simple model customization use Unit editor.
For advanced triggers you will need Jassgenpack. Jass/vJass gives much more options and has less limits than GUI.
However if you want to customize models significantly (or textures, icons) you will need few tools. Go to tools section, you will find many usefull ones.
I recommend Warcraft Model Editor first.
NOTE: Go to tutorial section, there a tons of great stuff that will help you quickly get familiar with spells/models. However, everyone has to practise to became a "master".