I recently installed Ubuntu and Debian for the first time. Here's how I did it.
The first thing I did was order a second 2.5" hard drive. I chose 320 GB drive, which is the largest size you can get without Advanced Format. I figured that I didn't need tons of space and I didn't want to worry about partitioning issues with an Advanced Format drive. My laptop is a Thinkpad T400. The great thing about Thinkpads is their modular design. Normally, the T series Thinkpads come with a DVD drive, but it is removable. So I took out the DVD drive and swapped it for a second hard drive. I found a third party hard drive adapter for the Thinkpad Serial Ultrabay Slim form factor. This way, I was able to keep my Windows 7 install on my main hard drive and not worry about breaking it with a Linux install.
I followed this partitioning guide and used GParted to split the second hard drive into four partitions: two root partitions (30 GB each for Ubuntu and Debian), a swap partition (8 GB), and a share partition (230 GB). I used Tuxboot to install GParted on a bootable flash drive.
Everything went great up to this point. Following the recommendation on the Ubuntu install page, I used Pen Drive's Universal USB Installer to load up Debian and Ubuntu on bootable USB drives. Then I tried to install Debian Squeeze and it kept failing to find my wifi. I tried my wired connection, but that failed, too. Since I was doing a net install (minimal Debian) off of a USB flash drive, I needed an internet connection to download all the GUIs and other packages which make Debian usable. However, because I had no internet connection, I kept ending up with a "command line" style Debian. I even tried downloading the "non-free" Debian which contains software that hard core Debian people consider proprietary. Didn't work. Ugh. After trying to get this to work all night, I gave up and installed Ubuntu 12.04 LTS on my other root partition. It booted up fine the first time, but the second time, I got a black screen. I finally went to sleep as the sun was rising.
When I woke up, I booted up Windows and remembered that I forgot to change my BIOS settings for the graphics card. My Thinkpad has switchable graphics, meaning that it can switch between integrated graphics and the discrete graphics card. I turned this off and set my computer to always use the discrete graphics card. Then Ubuntu booted up correctly and consistently. As for Debian, I went to the #debian IRC channel and presented my problem. People told me to avoid the current stable release ("Squeeze") and use the testing release ("Wheezy"). In addition, they told me to use the non-free version of Wheezy and to make sure I had all the firmware files for my wifi card. When I followed these directions, Wheezy found my wifi card and installed the packages off the internet. Now I had a desktop for Debian. Finally, I had both Ubuntu and Debian working!!!
The last thing to do was to fix my GRUB screen. The Ubuntu and Debian installers are really smart and automatically added themselves to the GRUB screen, allowing me to triple boot in Windows 7, Ubuntu, and Debian. However, after installing two distros and updating Ubuntu, my GRUB screen contained many useless entries. So I installed the GRUB customizer package on Ubuntu and used that to clean up my GRUB. Here's my edited GRUB screen: