Let’s get this out of the way: it’s really hard to tell the disciples apart. They have similar names, they’re all lumped together usually, some are barely even mentioned, etc., etc. The Illustrated Guide to the 12 Apostles by Jeffrey Kranz and illustrated by Laura Converse Kranz is a free (!) e-book which seeks to solve some of these difficulties.
Each apostle has a pretty neat sketch drawing depicting them (which is largely conjecture), along with several icons and other images which help to distinguish between them. These are each individually helpful, and the icons and the like are based upon the few details mentioned about each apostle in the Bible.
The work surveys all the information we know for certain about the apostles as found in the Bible, while also noting other details we know from tradition and the like. For example, John is one of the most prolific writers of the Bible, as he authored Revelation, John, and 1-3 John as well. These details are extraordinarily helpful at distinguishing the various apostles from each other, and provide some sort of way to remember them as individuals rather than as a group.
The book is brief (55 pages) but packed with useful information. It’s a quick read that can also serve as a handy reference book related to the apostles. I highly recommend it, particularly at its (free) price. The book may be downloaded here. I would also very highly recommend you check out The Overview Bible, a web site dedicated to bringing general Bible knowledge to all. It’s very informative and the posts are all great reads.