Here are my thoughts on start-up or re-start... & fundamentals
1- Take a lesson from a certified casting instructor, this will accelerate your learning curve and prevent/minimize many errors that take a long time to correct. It also gives a "base" for other learnings and experimentations. Winter is a good time to do this...
2- Use the gear you have and stick with it for at least 30-60 days of fishing/practice time. It is so tempting to keep changing gear given the vast array of stuff that is out there. Once you have this gear relatively mastered, you can then appreciate the differences that alternatives offer. E.g. until you you get to "feel" the load on your current gear, testing other rods makes little sense.
3- If you only have a floating line, then buy a sinking leader that you can add to the front end of your floating line. This will allow you to learn to cast with a slightly heavier front end and also get down to some of the fish you are targeting. It is also much less expensive than buying another line and spool.
Casting is but 5% of fly fishing, but unless you become reletaviley competent (doing a 35-45 foot cast consistently with a good loop and rod/line load with a dry line) at that part first, the rest will be much more difficult, e.g. presentation, leaders, tippets, line management, specialty casts, etc.
Ditto as well on what DeerHaawk has said.
Pete AKA Frenchcreek from Calgary