Attribute Guide

Site with all attributes: http://minecraft.gamepedia.com/Attribute

val attributeItem = <minecraft:dirt>.withTag({AttributeModifiers:[{Operation:0,UUIDLeast:1,UUIDMost:1,Amount:5.0,AttributeName:"generic.movementSpeed",Name:"Speed"}]});

Operation: 0 is additive, 1 is multiplicitive. For the speed dirt example, 0 adds five to the speed, but 1 multplies the speed by five.
UUIDLeast/most: ignore. Not important unless you're using modifiers. Which you shouldn't for individual powerful items.
Amount: The amount of attribute added.
AttributeName: the name of the attribute being added, based on the wiki.
Name: name of the attribute.

NOTE: If using the item as an ingredient, you need to use .onlyWithTag along with .withTag.
Also note: the quotes around the attribute and the name are neccesary. You will get an error without them.