My startup disk is "almost" and sometimes full, HD space fluctuates between 500 and 900GB used?

Question

so I bought an HD big enough to prevent this happening but I get the on error that makes me quit all app on a daily basis.  I check on info and says that HD sometimes has nearly 900GB used and other times 500 and bit, in both ocasions have checked after the error appears and despite the difference between available space forces me to quit all applications anyhow.
1.-  I dont understand why the massive fluctuation!?
2.-  I resist to believe that you must keep less than 500 GB in your hard drive if you have bought a more expensive HD (1T) so surely there must be something wrong.
3.- Admitely am a newby to mac so half the posts already answered here do not make any sense to me as they are very high spec with terms am not yet familiar with, if you can help me using basic guidelines and steps to follow you would make my month as I cant even work here anymore, am about to despair, have tried all possible options including the failed Mckeeper which I had to remove with Mac cleaner application in the end, Onyx and I review my files constantly, emptying the bin etc except checking if any email was been sent countless times and stayed stuck somewhere in my system as I read somewhere because I just dont know how to do that, it might be an option.
I also have an annoying pop up from BT showing up every now and then as well which tell me I have given permission and which havent got a clue how to cancel, this makes me wonder if is there something like a virus trojan or similar mac equivalent on my system causing all of this problem.
I'm sorry that I migrated to avoid these kind of problems but found a lot easier to run an antivirus on my pc than trying to solve a mac problem! Please help me

Answer 1

First, empty the Trash if you haven't already done so.
Use a tool such as OmniDiskSweeper to explore your volume and find out what's taking up the space.
Proceed further only if the problem hasn't been solved.
ODS can't see the whole filesystem when you run it just by double-clicking; it only sees files that you have permission to read. To really see everything, you have to run it as root.
Back up all data now if you haven't already done so. No matter what happens, you should be able to restore your system to the state it was in at the time of that backup.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the page that opens.
After installing ODS in the Applications folder, drag or copy — do not type — the following line into the Terminal window, then press return:
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up.
I don't recommend that you make a habit of this. Don't delete anything while running ODS as root. When you're done with it, quit it and also quit Terminal.