I have been playing Minecraft with ML for several years, but I have not used Forge before. I followed the Wiki's installation tutorial, and here is the error I have recieved:

 

 

2012-09-26 09:35:04 [iNFO] [ForgeModLoader] Forge Mod Loader version 3.0.196.366 for Minecraft client:1.3.2, server:1.3.2 loading
2012-09-26 09:35:05 [iNFO] [sTDERR] Exception in thread "Thread-5" cpw.mods.fml.relauncher.ReflectionHelper$UnableToFindClassException: java.lang.ClassNotFoundException: net.minecraft.client.Minecraft
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.ReflectionHelper.getClass(ReflectionHelper.java:159)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.FMLRelauncher.setupNewClientHome(FMLRelauncher.java:110)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.FMLRelauncher.relaunchApplet(FMLRelauncher.java:231)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.FMLRelauncher.appletEntry(FMLRelauncher.java:200)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at net.minecraft.client.MinecraftApplet.init(MinecraftApplet.java:28)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at net.minecraft.Launcher.replace(Launcher.java:143)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at net.minecraft.Launcher$1.run(Launcher.java:87)
2012-09-26 09:35:05 [iNFO] [sTDERR] Caused by: java.lang.ClassNotFoundException: net.minecraft.client.Minecraft
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:125)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at java.lang.Class.forName0(Native Method)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at java.lang.Class.forName(Class.java:264)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.ReflectionHelper.getClass(ReflectionHelper.java:151)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	... 6 more
2012-09-26 09:35:05 [iNFO] [sTDERR] Caused by: java.lang.NoClassDefFoundError: net/minecraftforge/event/Event
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at net.minecraftforge.transformers.EventTransformer.buildEvents(EventTransformer.java:55)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at net.minecraftforge.transformers.EventTransformer.transform(EventTransformer.java:35)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:167)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:118)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	... 11 more
2012-09-26 09:35:05 [iNFO] [sTDERR] Caused by: java.lang.ClassNotFoundException: net.minecraftforge.event.Event
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:125)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	... 15 more
2012-09-26 09:35:05 [iNFO] [sTDERR] Caused by: java.lang.NullPointerException
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at org.objectweb.asm.ClassReader.<init>(Unknown Source)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.common.asm.transformers.SideTransformer.transform(SideTransformer.java:27)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.RelaunchClassLoader.runTransformers(RelaunchClassLoader.java:167)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:118)
2012-09-26 09:35:05 [iNFO] [sTDERR] 	... 17 more

 

i'm not 100% but this post should go into the forge support section. Also try installing just Forge into the Minecraft.jar using a clean Minecraft.jar.

You could also check out MultiMC. It's a great little program to more easily handle all the mods and can also keep separate minecraft instances.