I use a NailTek file (from Ulta). It's glass and comes in a mini for my purse and and larger size. It's a pretty fine grit, so not great for taking off length (although it will), but great for maintaining length.
Be sure your nails are dry. Damp/wet nails damage easier when filing. Also, filing with clear base coat, at least, protects them. Some people prefer to file with a darker polish to get an even shape. I can't do it that way. I file better with naked or clear polish.
Filing one direction takes some getting used to, you tend to drag down. Try angling the filing hand, rather than angling the file. I file mine when I'm watching tv a lot, so I'll lay my hand on my leg and then file perpendicular at a right angle. Or I hold my hand flat, fingers pointing toward the ceiling, and file at a right angle. Then file the corners. Check the back side (palm side frequently) for eveness.
Loodie has a bunch of posts on filing
http://loodieloodieloodie.blogspot.com/search/label/Nail%20Files