The best online resource would be yerushalmionline.org. It contains shiurim from R' Yosef Gavriel Bechhofer on the entire Yerushalmi, and links to helpful seforim.
The top things to keep in mind (sorry, I can't think of exactly 5 right now) are that Yerushalmi's language is more terse than that of Bavli, and there is often little or no consensus as to how a particular passage is to be interpreted. Without Rashi, many aspects are hard to pin down with any degree of certainty.
I'm not aware of any commentary from a Rishon directly on Yerushalmi, other than two on Shekalim (which is commonly treated as a "Bavli" masechta). However, there are various compendiums that collect the words of the Rishonim according to the order of Yerushalmi.
Artscroll is currently in the middle of elucidating the Yerushalmi, in the same style as their Bavli elucidation. Other than that, Rav Chaim Kanievsky has a to-the-point commentary that is currently available on Zeraim and Moed. (You can find it on Hebrewbooks.org, but they unfortunately did not enter the title or author information properly. You'll need to search for תלמוד ירושלמי, and the seforim that were printed in Bnei Brak are the ones you're looking for.) There is also a multi-volume commentary called Lev Yerushalayim that I would recommend, but I don't think it's available online.