A first great tool is W32DSM89. It is a powerful disassembler. Disassembling a program is reversing the .EXE file into a file full of assembler instructions. That's why it is important you have knowledge about assembler! Another disassembler is IDA Pro, but in my (modest) opinion IDA is a too complex disassembler to start with.

A second indispensable tool is SoftICE. It is an incredible debugger made by Numega. The main advantage of SoftICE is that is remains resident in memory so you can pop it up everywhere and anytime you want! It doesn't slow down your system and it's compatible with almost every program or game. A MUST have!

Third important tool: a hex editor. Its purpose: changing a .EXE when you cracked it. Don't bother if you don't understand what I mean, it 'll become clear later on. I use 2 hex editors: HIEW (or Hackers View) and Hex Workshop. HIEW is a very very powerful DOS hex editor (I use it if the changes I have to make are whole instructions) and Hex Workshop is a powerful and easy to use Windows hex editor. But it's not as powerful as HIEW.

Fourth tool is RegView. It is used for searching the Windows registery.

Fifth tool is Windows Commander. It's a Norton Commander look-a-like but for Windows. Windows Explorer won't do because you can't copy a file, ex.: TEST.EXE, to the file TEST.BAK in the same directory.

And last but not least Smartcheck! It is a program developed by the Numega company (yea, the same as SoftICE). It's used to check errors in Visual Basic programs, but it's commonly used by crackers to crack Visual Basic programs. I'll put it online very soon as it's kinda difficult to get. If you download it, don't forget to rename the .piz into .zip!
Soon I'll write a tut on how to crack with it...for now you can find out on the pages in the links section.

Okay that's it. Now where can I get them? SoftICE, W32DSM89 and HIEW can be downloaded at http://protools.cjb.net or somewhere at http://www.crackstore.com The programs there should be registered, if they aren't just mail me or search for a crack at http://astalavista.box.sk (best cracks site!!)
Hex Workshop can be downloaded at http://www.bpsoft.com RegView 1.21a can be found at not yet found! And finally Windows Commander can be found at http://www.ghisler.com