Could you open your finder and let us know how much spare hard drive space you have on the main disk.
If you have issues just click Finder in the Dock, Click Computer in the Finder window, Select the disk you want to examine and Choose the Show Info command from the File menu.
Also is this a desktop or notebook?
Let us know
In your Finder select your startup disk's icon. which should be Macintosh HD.
Press the Command+I keys.
The Get Info window for your startup disk should open and in the General pane, the Capacity, Available (free space), and space Used on your startup disk will be displayed.
Let me know what it shows.
When you open the finder do you see anything that says Macintosh HD?
If so then that is your startyp disk, just give it a single click and then press the Command and the I key together which should open the Info window so you cn follow the instructions I posted.
Let me know
Do you see a screen like this when you open your finder?
What icons do you have on your desktop?
Do you see one named Macintosh HD?
I am going to opt out and see if another Mac expert can help you as If you cannot search or browse to your drives I cannot help.