Having problems with my Macbook Air startup disk?

Question

I bought this laptop last spring and lately it's been giving me some problems.
First of all, my startup disk is already full. I only use this as a home computer for schoolwork, photos, music, etc. Not sure if the reason it's full is because of pictures. Does anyone happen to know how many pictures you can out on a Macbook until it's maxed out?
Also, when I try to update the software it tells me I need more storage space. So I tried to get into iPhoto to delete some pictures but it says I don't have permission to "write to photo library."
I apologize if this is too general of a question but I figured I'd ask here before calling Apple for help.
Thanks for any tips you can give me in advance!

Answer 1

Back up all data.
This procedure will unlock all your user files (not system files) and reset their ownership and access-control lists to the default. If you've set special values for those attributes on any of your files, they will be reverted. In that case, either stop here, or be prepared to recreate the settings if necessary. Do so only after verifying that those settings didn't cause the problem. If none of this is meaningful to you, you don't need to worry about it.
I've tested these instructions only with the Safari web browser. If you use another browser, they may not work as described.
Step 1
If you have more than one user account, and the one in question is not an administrator account, then temporarily promote it to administrator status in the Users & Groups preference pane. To do that, unlock the preference pane using the credentials of an administrator, check the box markedAllow user to administer this computer, then reboot. You can demote the problem account back to standard status when this step has been completed.
Triple-click anywhere in the following line on this page to select it:
{ sudo chflags -R nouchg,nouappnd ~ $TMPDIR.. ; sudo chown -R $UID:staff ~ $_ ; sudo chmod -R u+rwX ~ $_ ; chmod -R -N ~ $_ ; } 2> /dev/null
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window (command-V). I've tested these instructions only with the Safari web browser. If you use  another browser, you may have to press the return key after pasting.
You'll be prompted for your login password. Nothing will be displayed when you type it. You may get a one-time warning to be careful. If you don’t have a login password, you’ll need to set one before you can run the command. If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator.
The command may take a few minutes to run, or perhaps longer if you have literally millions of files in your home folder. Wait for a new line ending in a dollar sign (“$”) to appear, then quit Terminal.
Step 2 (optional)
Take this step only if you have trouble with Step 1 or if it doesn't solve the problem.
Boot into Recovery. When the OS X Utilities screen appears, select
Utilities ▹ Terminal
from the menu bar. A Terminal window will open.
In the Terminal window, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window will open. You’re not going to reset a password.
Select your boot volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Under Reset Home Directory Permissions and ACLs, click the Reset button.
Select
 ▹ Restart
from the menu bar.