how do I delete files from my start up disk???

If your disk is 80% full that is normal. Time Machine uses up to 80% of the disk space for local snapshots. To get rid of these snapshots simply plug in your Time Machine backup drive and run a backup. See About Time Machine's 'local snapshots' on Mac notebooks: http://support.apple.com/kb/HT4878 and What are Local Snapshots? http://pondini.org/TM/30.html .

If you are concerned that the “Other” category of disk usage is taking too much space and for information on deleting files then look here: https://discussions.apple.com/docs/DOC-5142

If you have less than 20% disk space then it is time to roll up your sleeves and search for what you can delete and what you can offload to another disk.

If you have less than 10 GB you definitely need to delete or offload some files or purchase a lager disk or SSD (see below). You may want to maintain at least 20 GB of free space so when your disk starts filling again it will have some room before it hits that 10 GB mark again. More headroom is better. If you let the space fall much below 9 GB you might not be able to boot your machine.

Backup:

Run a Time Machine (or other) backup since you are about to delete and move files and you may need to recover from any inadvertent mistakes or decisions. You will need one external hard drive for your Time Machine (or other) backup and a second if you plan to offload some files. (See suggestions for where to purchase hard drives at the end of this message.)

For more about backups:

Time Machine Basics: http://support.apple.com/kb/ht1427

Most commonly used backup methods:

https://discussions.apple.com/docs/DOC-3045

Methodology to protect your data. Backups vs. Archives. Long-term data protection:

https://discussions.apple.com/docs/DOC-6031

Initial easy steps to gain disk space:

- Delete all files in the Downloads folder.

Empty the Trash.

- Start iPhoto.

Empty its trash.

Restart.

- Delete "Recovered Messages", if any.

Hold the option key down and click "Go" menu in the Finder menu bar.

Select "Library" from the drop down menu.

Library > Mail > V2 > Mailboxes

Delete "Recovered Messages", if any.

- Empty the Trash.

- Restart.

- Re-index Macintosh HD:

http://support.apple.com/kb/ht2409

- Repair Disk

Steps 1 through 7:

http://support.apple.com/kb/PH583

Deleting files:

Then use the free application OmniDiskSweeper http://www.omnigroup.com/more to explore your volume in descending order by size so you can attack the problem from the top down, deleting the largest unwanted files first. Delete with caution and do not delete any system files. Remember to empty the trash after trashing the files.

Additional reference on freeing disk space:

http://pondini.org/OSX/DiskSpace.html

Offloading files:

Consider moving some of the no-often-used large files or directories to an external disk. Use ODS again to find them. As noted above this will be at least your second hard drive. Your first one(s) is/are for your Time Machine (or other) backup(s). Do not offload files onto a Time Machine disk.

Format the second drive as Mac OS Extended (journaled). Using OWS to find large files/folders and copy them from the system drive to the external hard drive and delete them from your internal drive.

Then  > System Preferences > Time Machine > Options… > Remove the offload HD name from the exclusions list.

Now both your system disk and your external offload disk will be backed up onto your Time Machine disk.

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From: http://www.thexlab.com/faqs/freeingspace.html

To move your iTunes Music folder to another disk or partition:

To change the location of your iTunes Music folder, carefully follow the instructions in the AppleCare® Knowledge Base document "iTunes for Mac: Moving your iTunes Music folder."Additional information can be found in iTunes Help.

http://webcache.googleusercontent.com/search?q=cache:http://www.thexlab.com/105/ 00000849.html

Laptop users may want to consider having two iTunes libraries: a small library of current favorites on their computer, while their complete library resides on an external hard drive. Utilities like iTunes Library Manager enable you to easily have multiple iTunes libraries you can use with your account. https://www.macupdate.com/app/mac/7689/itunes-library-manager

To move your iPhoto Library folder to another disk or partition:

To move the iPhoto Library folder to a new location, employ the instructions in the AppleCare Knowledge Base document from http://support.apple.com/kb/PH2506 corresponding to the version of iPhoto you are using. Additional information can be found in iPhoto Help.

Laptop users may want to consider having two iPhoto libraries: a small library of current, favorite photographs on their computer, while their complete library, or archives of older photos are saved on an external hard drive. Utilities such as iPhoto Buddy and iPhoto Library Manager enable you to have multiple iPhoto libraries that you can use with your account.

https://www.macupdate.com/app/mac/12175/iphoto-buddy

https://www.macupdate.com/app/mac/7158/iphoto-library-manager

Hardware — Bigger disk/SSD:

If your system has upgradeable storage then if you are still tight on disk space consider larger storage. If you have a disk consider replacing it with a one TB disk. Check out a one terabyte HGST 7K1000 7200 rpm, SATA III drive from OWC http://eshop.macsales.com ($100). If you have an SSD consider increasing its capacity to 240 or 480 GB. A standard 240 GB SSD would cost from $200 to $280. See OWC and Crucial: http://www.crucial.com/ for options. OWC sells 120, 240 and 480 GB SSD upgrades for 2009 to 2012 MacBook Airs. A 240 GB upgrade costs $265. OWC also sells a 120 GB SSD upgrade for 2008 models for $155. http://eshop.macsales.com/shop/SSD/OWC/ . If your Mac is under warranty or AppleCare replacing the SSD will void the warranty.

PlotinusVeritas gives some great suggestions for purchasing external hard drives in this thread:

https://discussions.apple.com/thread/5602141?tstart=0

You need to start from the beginning.

Although it may not make any difference we need to know what kind of files you deleted. If you know what is actually filling up the drive we may need to know that too, (the filenames and where they're located).

Secondly you should back up anything important.

Then lets start again from a cold boot with nothing open and see if anything is happening there.