how to fix a poor spackle job?
Get yourself a drywall sander



Sand down the high spots using rough grit (100 grit) sand paper (they sell it to fit the sander - you'll find the sander in the drywall area and the sandpaper in paints) then smooth it with 200 grit.

If you have low points, fill them in after sanding.

Next time use less mud (little applications, not big ones) and then use a really really big sponge that's been slightly dampened to even it out.