Go here:
http://customgraph.com/SG/piart.php?art=229
Print out this graph paper it is 1/4" size. After printing, double check with your ruler just in case there is some scaling issues with your printer.
Take the paper and bring your needle down on any line on the graph paper. Look at where the next line is to the right. That is where your 1/4" is on your sewing machine. Now do what CK Cowl advised and put down masking tape. Take a very find sharpy marker and also mark a little line on your presser foot so you know where the fabric needs to go under your presser foot as well, to maintain the 1/4" seam allowance.
The whys have been explained by previous posters. You really want to master a 1/4" seam allowance so you can make any pattern you come across without having to redraft or be concerned about bulk. If you seams are coming out you need to shorten your stitch length. You can do this!