Grinding when downshifting into 1st

Hey guys,
I have an issue with the mustang, and I'm not the best mechanically. One of my buddies mentioned the synchro ? Anyway, the problem is that you cannot shift the car into 1st unless you are absolutely stopped, and I mean STOPPED. It grinds if you try to put it in at all, if the car is rolling, if your downshifting at a stop, pretty much all the time unless completely stopped in traffic for at least a few seconds, then you can put it in without a problem. I know it isn't good to downshift into 1st anyway, but this goes beyond that, and regardless the car shouldn't do this. Is this the synchro? What is the synchro, and is it expensive/difficult to fix? Could this be anything else? Car also grinds a bit into second if it is freshly started and run at a high rpm (pushed). Other than this the transmission seems fine? Thanks guys/girls
-Jon

thanks guys, and i know that i shouldnt be downshifting into first. i dont even do this. the problem is that i CANT shift it into first even if i wanted to unless i am seriously dead stropped for a few seconds, and then i throw it into second then quickly up into first. I am not saying i want to downshift, but the whole shifting into first at a slow roll is not even an option. I am looking to sell the car at some point, and whoever buys it will not be happy about the situation, it is real noticeable, and prevents you from shifting into first unless dead stopped. i just need to know what the problem is, what would cause this, and is it a big deal to fix? if you drove my car you would see what i mean. the first few times you would normally throw it into first, it would grind so bad you wouldnt be able to get it in. its real finicky. thanks again
-jon