Installing Homebrew: .git already exists

I should mention that I have no idea what I am doing 🙂 So, sorry if this is confusing.

I am trying to install Homebrew with this command

/usr/bin/ruby -e "$(/usr/bin/curl -fsSL https://raw.github.com/mxcl/homebrew/master/Library/Contributions/install_homebrew.rb)"

using the directions here https://github.com/mxcl/homebrew/wiki/installation

After installing i ran brew doctoras it said to, I got these errors:

    Error: /usr/local/include isn't writable.
This can happen if you "sudo make install" software that isn't managed
by Homebrew.

If a brew tries to write a header file to this directory, the install will
fail during the link step.

You should probably `chown` /usr/local/include
Error: /usr/local/lib/pkgconfig isn't writable.
This can happen if you "sudo make install" software that isn't managed
by Homebrew.

If a brew tries to write a .pc file to this directory, the install will
fail during the link step.

You should probably `chown` /usr/local/lib/pkgconfig
Error: /usr/local/share isn't writable.
This can happen if you "sudo make install" software that isn't managed
by Homebrew.

If a brew tries to write a file to this directory, the install will
fail during the link step.

You should probably `chown` /usr/local/share
Error: Some directories in /usr/local/share/man aren't writable.
This can happen if you "sudo make install" software that isn't managed
by Homebrew. If a brew tries to add locale information to one of these
directories, then the install will fail during the link step.
You should probably `chown` them:

    /usr/local/share/man
    /usr/local/share/man/man1
Error: The /usr/local directory is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. Some versions of the
"InstantOn" component of Airfoil are known to do this.

You should probably change the ownership and permissions of /usr/local
back to your user account.
Error: You have no /usr/bin/cc.
This means you probably can't build *anything*. You need to install the Command
Line Tools for Xcode. You can either download this from http://connect.apple.com
or install them from inside Xcode's Download preferences. Homebrew does not
require all of Xcode! You only need the Command Line Tools package!
Error: Unbrewed dylibs were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected dylibs:
    /usr/local/lib/libavcodec.52.11.0.dylib    /usr/local/lib/libavcodec.52.dylib    /usr/local/lib/libavcodec.dylib    /usr/local/lib/libavformat.52.25.0.dylib    /usr/local/lib/libavformat.52.dylib    /usr/local/lib/libavformat.dylib    /usr/local/lib/libavutil.49.14.0.dylib    /usr/local/lib/libavutil.49.dylib    /usr/local/lib/libavutil.dylib
Error: Your compilers are different from the standard versions for your Xcode.
If you have Xcode 4.3 or newer, you should install the Command Line Tools for
Xcode from within Xcode's Download preferences.
Otherwise, you should reinstall Xcode.
Error: /usr/bin occurs before /usr/local/bin
This means that system-provided programs will be used instead of those
provided by Homebrew. The following tools exist at both paths:

    idle
    idle2.5
    pydoc
    pydoc2.5
    python
    python-config
    python2.5
    python2.5-config
    pythonw
    pythonw2.5
    smtpd.py
    smtpd2.5.py

Consider amending your PATH so that /usr/local/bin
is ahead of /usr/bin in your PATH.
Error: Your Xcode is configured with an invalid path.
You should change it to the correct path. Please note that there is no correct
path at this time if you have *only* installed the Command Line Tools for Xcode.
If your Xcode is pre-4.3 or you installed the whole of Xcode 4.3 then one of
these is (probably) what you want:

    sudo xcode-select -switch /Developer
    sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer

DO NOT SET / OR EVERYTHING BREAKS!

In trying to fix these errors, I ran
git clean -n
git clean -df
git init
then git clean -df again.

So then when I ran brew doctoragain, it said -bash: /usr/local/bin/brew: No such file or directory.

After that, I tried to reinstall Homebrew but now I get /usr/local/.git already exists!.

So, how do I fix these problems and get Homebrew installed?

Thanks in advance and sorry for my confusion.