How can I attach an under-sink water filtration system to the water line?
You're going to need to get a 3/8" x 3/8" x 1/4" brass tee, like this one from Home Depot.



Then you'll simply screw the tee onto the male end of the shutoff valve (where the cold water supply line should currently be attached). Attach the supply line (that you removed from the shutoff valve) to the male end of the tee. Finish the installation by attaching the tubing to the 1/4" outlet using a compression fitting (Which should be included with either the filter or the tee).