I did the same thing when I bought my Washburn DB32 2 years back on the low e area. First find a nut that goes with your guitar online or in a store. Not every nut is the same. If you're feeling gutsy, you can actually buy a blank nut and file the grooves yourself, but I wouldn't recommend that seeing how you've never done it before. Then you need a wooden mallet, some crazy glue or any superglue, and maybe a file or very fine sand paper to fix the area below the old nut if needed. On most guitars, you really have to just tap the side of the nut with the mallet and it should just pop off. A strat might be different though(you might need a flathead screwdriver or something). Then you make sure that the area is fairly clean without any bumps or anything. After you inspect the area, apply a little bit of crazy glue on the sides between the truss rod hole(if its there of course, if not then just apply it across). After you have a decent amount there, then put the new nut on and hold it from the top and line it up until the glue dries which shouldn't be more than a minute. You should be good after that. Just make little adjustments to the groove according to the string with a thin file.
Of course, it this all sounds foreign to you, just take it to an experienced guitar tech someone you know that can fix it for free. A tech shouldn't charge any more than $20-$40.
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