Your start up disk is the internal HD inside your MBP, apparently you have filled it up with data. If you use it to store music, photos and other data eventually it will become full. Additionally you probably should empty your trash from time to time, just like your home if you keep throwing thing into the trash eventually you need to empty it. You will either have to delete data files you no longer need or begin moving them to an external HD. Mac's need about 10-15% of the disk free to operate efficiently, if you ignore the message this is not good for your machine. External HD's are inexpensive these days and moving data files to them is a simple drag and drop operation.
Your start up disk is the internal HD inside your MBP, apparently you have filled it up with data. If you use it to store music, photos and other data eventually it will become full. Additionally you probably should empty your trash from time to time, just like your home if you keep throwing thing into the trash eventually you need to empty it. You will either have to delete data files you no longer need or begin moving them to an external HD. Mac's need about 10-15% of the disk free to operate efficiently, if you ignore the message this is not good for your machine. External HD's are inexpensive these days and moving data files to them is a simple drag and drop operation.
A few things though. Where do i find where everything is. I have done a search looking for large sized files and not much came up, and my 360GB hard drive is full. I have a 18GB partition, so is it stuff like Iphoto? I am just looking for places things can be as i don't have a massive amount on here. Any direction would be great.
Download and run the freeware Disk Inventory X. It will show you exactly what is using all the space on your hard drive, including files you can't ordinarily see. Post back if you have further questions about what it shows you.