If you are going to be printing on it with a solvent printer (Roland VP540 or 300), then you should go with one of their digital print lines (3164 or 3165). Otherwise, I'd start with Oracal 651 to learn with, it's a decent vinyl and cheap, so when you make mistakes (and you will, it's a trial by fire business) it doesn't hurt the pocketbook too much. Many people still stick with 651 unless they're getting into things such as vehicle wraps, which requires a few more fancy features. I usually order from
Ordway Sign Supply - The Complete Source for Today's SignMaker. but I have also dealt with
The World's Largest Wrap Supply Company - Fellers and
Sign Supply for Vinyl Cutters & InkJet Printers - Beacon Graphic Systems before as well without any issues.
App tape, with bigger letters like you're doing, I'd probably go with a medium tack transfer tape. A lot of people swear by R-tape's clear AT60, haven't used it yet, but will be getting a roll soon as I'm going to be doing some stickers that customers will actually apply (much easier for them to line up when they can see through it).
I usually use TransferRite high or medium tack, high tack when I have smaller graphics, medium for bigger ones. I used to apply wet all the time, but have started applying a majority of my decals dry now, since I had quite a few problems with transfer tape pulling letters up when wet.
I would also suggest checking out the Signs 101 forum
Signs 101 - Signs 101 Home I would not hastily post though, as a lot of the members there have been in the business for a long time and do not take well to people starting up on a whim per se. Seen many folks thrashed when they come in saying they're starting to do signs and want to know what to get, however there is still a lot of useful advice and people there! It's kind of like the old Linux days where the common response to everything was "RTFM", knowledgeable people, but they expect you to do the research and read before asking the questions.