startup disc is almost fulll

It means that the disk or partition that the disk boots from is almost full, so yes it could be the amount of stuff you have saved on your computer. If you have a separate partition you could move some of your stuff to there to free up some space. Also I believe there are some utilities that you can use to delete temporary system files to free space that way.

I would recommend you take a back up before you do either just to be on the safe side.
Backup your files? You should have an external hard drive for that. You can simply plug that in to your Mac, and your Time Machine software should (normally) ask if you want to use that external hard drive for a Time Machine backup. Or, you can open your System Preferences/Time Machine, and turn Time Machine ON, (you may have turned it off at some earlier time), and let Time Machine take care of your backup. The backup will likely take several hours.
There's other software options for backups, such as Carbon Copy Cloner, or SuperDuper!

This might also be a good time to consider upgrading/replacing your hard drive with a larger capacity...
How large is your hard drive now?
If you Get Info on your hard drive, you can see the capacity, and how much free space you have left (among other items.)
You will get a low space warning when free space dips below about 200 MB. You may also see that low space warning, even if you have more space.
Most experienced OS X users will tell you that you should keep at least 10% of total capacity as free space, and some even suggest up to 25% free. In these days with possible TeraBytes available, I'm not sure if you would ever need to keep 1 TB free on a 4 TB drive, even if a boot system - but more space is better.

You can also sometimes recover a lot of space by removing old log files, and various system and user cache files. There's good utilities that can make a cleanup of those various temp files very easy - such as OnyX, or YASU.
Monolingual is a great utility to free up space. Be VERY CAREFUL what languages you delete, though. It is not recoverable without a new system install. If in the USA, do not delete English or English (United States)---don't ask how I know!! :)
How much free space do you have on your hard drive?

If you need to back up files - then you need to get an external.
If you are presently traveling, then you could simply look at your larger files that might be on your hard drive, and see what you might delete. Remember that an item in the trash still takes up space on your hard drive, and you may get lots of space back if you have ignored emptying the trash recently. I recently had a customer system with 120 GB of files in the trash, because he though the trash didn't count against space used.

I like to use OmniDiskSweeper to get a good list of all your large files. It can also let you delete some large files directly from that app.
how do i find out how much space i have? nope always delete my trash can.. so maybe deleting some applications or stuff ive downloaded could help? is that app free?
OmniDiskSweeper is free -

SGilbert also mentioned Monoligual - also free -
That's a good one that can get back several GB of disk space, the first time you run it. It deletes all language support files, except for your local language.

Some apps - even though you may not use them - may not take up enough space to make it worth the effort to remove - some are just not very large. That's where the OmniDiskSweeper will help, showing you exactly how much space is used, and where.
Virtually anything is Caches folders can be deleted, and you will probably find out that some of the Caches folders are quite large.