What you might try.
"And can I delete appies I have never used (and really don't even know what the they do) like;
iWeb
GarageBand
front row"
I suspect that deleting those apps will not buy very much relief.
No doubt that Pig003 is correct in giving your whole approach to organizing your files is important. No doubt family photos and videos are important. As I am now 61 I find that many of the pictures my family had from years ago, and everyone swore that we would take care to keep, have been lost.
From what I read, the Journaled file system of OS X does not fragment to the point where it has problems. However, the one way to create a lot of fragmentation, and exacerbate the the problems in the amount of space you have left on the drive is to do a lot of video editing.
Before trying much of anything. Back up whatever you have, as no one can guess the true consequences to your drive of trying things which work OK normally.
While I feel you know about it already:
1. Empty the trash bin.
2. Try to "Repair Disk Permissions" which should not harm anything, but might. Repair Disk Permissions will not put more space on the drive, but it can speed up a computer quite a bit.
Not sure how to advise you to look for "Fragmentation Problems" After you move off to external storage large files from the main drive, some would suggest to back up the internal drive, then (and this is dangerous, because you could lose all of your private files) Then Format the internal drive again. Install Mac again. update it. Then bring over your personal files, and programs --
Actually I let one of the more knowledgeable folks here talk about the right way to bring your personal files, and chosen programs back onto to the active internal hard drive - so that the hard drive is effectively without fragmentation. I am not sure you would have fragmentation problems to the extent of causing delays after some open space is gained by file deletion on the internal hard drive. However, if one is a bug about performance, then getting enough space, and getting the drive without fragmentation would seem important to me. Not sure if it is worth the effort for some folks tho.
Also having been a kid whose mom threw away some of my things. I know how upset I got when I discovered she did it. Might keep a copy of the kids music on an external drive or something.
USB Flash drives are neat and getting less expensive, but I like to use a spinning hard drive in an external enclosure, (or its sealed equivalent). In fact one can boot from such a drive, just my computer does not run so fast with the OS on a USB HD. Booting from an external drive is a cool way to work on building a HD just the way I want it, before I give up my computer as is, or boot from external drive to work on some problems.