Within the "Something Else" installation option, here is what you want to do:
1) Select the current Windows partition (/dev/sda1).
2) Click "Change..."
3) You will need to shrink your Windows partition to make room for a Linux partition. Enter a new size for your Windows partition. Based on how much space you've used (almost 30 GB), you could safely shrink it down to 60 GB (60000 MB) and have plenty of room to spare.
4) After the /dev/sda1 partition has been shrunk, you will now see a new option in the list: "free space". Select this option, then click the plus icon to add a new partition in the free space, and give it the following options:
Type: Primary
New partition size: just accept whatever's here (it will be all your available free space)
Use as: Ext4 journaling file system
Mount point: / (you can select it from the drop-down menu)
Then click OK. Your new partition will show up as "/dev/sda2" alongside your Windows partition on "/dev/sda1", and you can proceed to "Install Now".
Good luck
