Lock yourself in your home this weekend and spend every hour possible on YouTube learning how to use a particular type of software commonly used by ecommerce companies (eg. Kiss Metrics, CrazyEgg, Click Tales, Google Analytics, Google Conversion Optimizer, Google Tag Manager, etc...).
The goal is to consume as many hours of video on a particular tool type as possible. And don't worry about the quality of that information just yet - after about an hour or two, you'll start to build a reference for what's good/bad in terms of content.
Also, take out a new notebook that you can solely dedicate to notes on that topic. It will be your field guide. Carry it everywhere, and study your notes during breakfast and before bed.
Keep in mind that the goal here isn't to become the "expert" - but rather get to a point where you know more than your potential client or the people on their team responsible for managing those tools (so you can offer value). You'll be surprised how 40-50hrs of video can accelerate that for you since most people managing accounts day-to-day rarely get that time.
Next, focus on your offering. Something free.
For example, let's say you start getting really strong with Google Tag Manager and discover a few Chrome plug-ins that will tell you if a tag is broken. You could then use that to check the sites of prospective clients and give them a free troubleshooting report of the issues they have (your service would be either a more extensive report, or your help in fixing those issues).
Also, there are some great tools out there to narrow down the prospect list based on technology. For example, you could use builtwith.com to find sites that already use Google Tag Manager and narrow down your outreach to them.
Holler with questions, and good luck!
