Where you start depends on where you've been. If you have systems that were successful in the past, but then got dropped for some reason, bring them back. Employees who remember the system will champion it and help spread it through the rest of the business. By incorporating and adapting systems that teams are familiar with, it avoids the whole idea that lean is just another fad. At my company we had a really effective problem solving system in our old Operational Excellence days. We revitalized this system in our new Lean Enterprise and feel that it's working because so many people had success with it in the past.
Another word of advice: Think big but start small. I heard of a company who tried to do their first value stream map implementation on their most complex, biggest dollar manufacturing line. Their reasoning was that it was the line that affected sales and profit the most. Hmm, makes sense right? Well, the team failed and their lean journey ground to a halt. The site management could have selected a smaller more manageable project as their first lean implementation. Make a small model value stream, show the rest of the cells that lean systems really work and then attack the bigger projects. It's not always possible but try to pick a value stream where you know the owner will really champion the lean concepts.
My last but perhaps most important bit of advice on launching lean implementation: develop objective measurement systems. You can't figure out whether you're winning or losing if you don't have an objective measure that tells you how you're doing. How can you really tell if you're lean systems are doing any good if you can't measure your performance? Make the measures as objective as possible - preferably out of an MRP system - this avoids any measurement bias.
There's no magic answer to the question of "where do I start my lean journey?" but hopefully I've given you some good pointers. I'd love to hear your stories or thoughts on this matter. Where did you start and how did it work out?