Your startup disk is your internal hard drive, unless you boot from an external hard drive. See this on how to increase free space: http://thexlab.com/faqs/freeingspace.html
Sometimes running OnyX can help free up some space ... but you do need space to download it: http://www.titanium.free.fr/download.php . Run the tasks on the cleaning and maintenance tabs.
You sound like a newbie Diane, so I'll answer from that assumption (no slight intended):
Best idea #1: Get a bigger harddrive. Problem solved. Unless, of course, a person doesn't "clean house" so to speak every once in a while so that the garbage builds up on their HDD until it is once again unnecessarily clogged with unknown 'stuff.'
Tips to get space back:
First, your "/Library/Printers" directory is probably 90%+ filled with hundreds & hundreds of megabytes of printer files that you will never, ever use. If you were to completely empty that folder, & then plug in a brand-new printer, eventually software update will pop up a message for you to download drivers for it, so there's really no need for multiple manufacturers' PPDs to be kept in the Printer folder. You're going to be surprised how much space you'll regain from dumping all the printer files you don't need. HUGE amount.
Also, dump old emails, especially ones with big attachments. You can do a sorting of your Inbox so that all the emails with attachments will show up at the top of the list. And if you have sent a Reply to a msg that's got a picture in it, & you didn't disable sending that attached picture in your Reply, then you will then have 2 copies of the same msg & attached picture: In the Inbox, & in the Sent box. Wasted space.....
Download & run Monolingual: http://monolingual.sourceforge.net/
This great little free util will do these things over about 10 minutes time:
1. Remove all things written in languages that is not your chosen language from inside applications on your HDD (instructions, readme's, etc);
2. Removes all languages but your chosen language from the Input Menu (Chinese, Korean, etc, if you're not of that ethnicity & read that language).
3. For the benefit of Intel machines, removes all PowerPC code from Universal Binary apps.
Also, get rid of applications you don't think you'll use in the next 6 months or so, & also pictures & videos taken with digital cameras that you don't *really* want to keep. All those things are big spacetakers.
From all these steps, you should see the recovery of 4 maybe 5 gigs of space.
One other idea: Get online & get yourself one of those 16 or 32GB flash drive & move stuff onto it. Those sizes of flash drives are at a very low price now.
Those tips are some good starter things you can do, for the time being, until you go get yourself a bigger HDD, Diane.
And by the way, with the exception of running Monolingual (it's automatic -- just select your language, deselect the rest, click its GO button, & come back in 10 minutes), REMEMBER THIS:
IF IN DOUBT, (AND I MEAN REAL WARRANTED DOUBT; I.E., BASED ON AT LEAST *SOME* KNOWLEDGE), NEVER FORGET THAT WHILE YOU'RE DELETING THINGS, YOU CAN ALWAYS CLICK ON THE CANCEL OR QUIT BUTTONS, OR TAP THE ESC KEY, ETC TO BAIL ON SOME DELETION PROCEDURE. COMMAND + PERIOD (.) WILL STOP MOST OPERATIONS DEAD IN THEIR TRACKS, TOO.
BETTER SAFE THAN "SORRY!."
I am a "newbie" as far as doing anything besides iphoto, itunes, etc.
This answer helped me solve another problem, but I can not seem to award any "points" to it. If you could tell me how to do it, I will certainly put them on there. Thank you for taking the time to write such a detailed answer.
Take care, Diane