Hi Goldilocks,
First a couple of points that you may not be aware of as this is your first Mac:
a) When you delete things by sending them to the trash they still remain on the computer until you empty it (in the Finder head to the menu also called "Finder" and drop down to "Empty Trash" ) This will permanently delete whatever is in the trash and thus free up some space.
b) Unix based operating systems like OSX run various "housekeeping" routines, usually in the wee small hours of the morning, to clean out things like "temp" files and the like. If your computer is not on, or is sleeping, at this time then they are meant to reschedule to a later time but sometimes this doesn't actually occur. You can use a program like
macjanitor or
Onyx to run these tasks at a time of your own choosing instead.
The software which comes pre-installed on your MacBook takes up quite a lot of space. You may want to think about whether you want to keep it all or not. You can always re-instal it from the DVd that came with your computer at a later date if you wish.
Some things that you might want to remove to free up some space if you don't use them:
1) The "trial" version of Microsoft Office
2) The "trial" version of iWork
3) GarageBand, part of the iLife suite, can be fun, but the templates which it comes with take up a lot of room. All up between its templates, instruments etc it consumes a couple of gigabytes of space. To free up this space you have to drag the application to trash and also drag the folder called "Garage Band " in HD/Library/Application Support to the trash (there are various "Library" folders on your Mac. This one is at the "root" level of your HD - ie the one that shows up in the window when you double click on your HD icon. (remember after putting things in trash you have to empty it to actually get the space back)
When you imported your music and your photos did you make copies of them on the HD and then import them into iTunes and iPhoto? If so it may be that you have two copies of each item on your computer - the ones that you copied across and the copies that were imported into these applications (which will be found in the home/Music/iTunes/iTunes Music folder and the home/Pictures/iPhoto Library folders) . Worth checking.
Its a good idea when using OSX to always keep several gigabytes free - at least 5 , preferably 10 or even more if you do things like video work. OSX uses space for memory "swap files" and also has to be able to write things like "extents" directory files. If the amount of free space gets too low then these processes can't work properly and you get the sort of awful slowdown you mention. Ultimately this can lead to a very nasty crash.
Cheers
Rod