I can't offer too much advice, but one thing that I know is harder than it looks is making perfect cuts. I think we all can do a rough dice, a rough julienne, etc. But try to make perfect squared dice--it is much harder than it seems. Until I saw my chef do it (and do it quickly) I thought a dice was just making squarish cuts. I didn't realize that perfect cubes were possibly when done by a great chef.
Lets not also forget that a julienne also has specific measurements (not just very thin slices). Symetrical rounds. Batonnets. Chifonades. Etc. By practicing your cuts, you will also make a lot of mise en place to then use in making your stocks. Which you can then use to make your sauces with. etc.
So from my POV, start with cuts.
