It really is getting quite easy to install Ubuntu onto a computer from Windows now – especially if you’re not trying to Dual-boot. But if you do want to Dual boot, you’d still do the following but you have to partition your hard-drive first (which is remarkably tricky).

Installall Ubuntu onto a USB stick

UNetbootin is your friend

Here’s the main Ubuntu document: https://help.ubuntu.com/community/Installation/FromUSBStick#From_Windows

1. Plug-in an empty USB stick
2. Download UNetbootin (http://unetbootin.sourceforge.net/) & Install
3. Run UNetbootin
4. Distribution: Ubuntu | Type: USB Drive
5. Click OK – wait for the download and the installation
6. Either Reboot when asked or click close and remove the USB stick

Install Ubuntu from USB-stick onto your computer

1. Set the computer to boot via USB, if you don’t know how, search Google for: [Your computer make & model] usb boot
2. Turn off the computer
3. Insert the USB stick
4. Turn on
5. Click Install Ubuntu