a quick and dirty crack sealer?

If you do a quick and dirty repair, it's going to look bad. Trust me, I tried it a few months ago. Get a good joint compound and a wide putty knife. If you've got large cracks you'll need tape too. A good multipurpose joint compound like this one should work.

I agree with Derek. Skills come with practice. So practice. Get a piece of scrap drywall, some joint compound, a 6 inch joint knife and go at it. Ask for some scrap wherever drywall is sold. Libraries have DIY books. The best of which are published by Taunton Press.

Derek and Buckeye are right, practice makes perfect.

For a crack that is not wide (<1/4 inch)and is shallow (no day light showing through the wall) get a small container of premixed spackle or wallboard compound and a small container of plaster of paris a flat surface to mix them on and a flexible 4" putty knife. You'll also need a sheet or two of sandpaper.

Drop a lump of spackle on your mixing surface and mix in a small amount of plaster of paris. The plaster makes it set up fast and hard so use it sparingly. Spread it on with your knife from top down feathering the edges to minimize sanding. Use multiple coats for deeper cracks.

Take your time and good luck.