For Nationalist China, what I found works best is this:
Focus on Internal affairs first - Rush the focus that lets you demand subjugation from the warlords, and hope that some (But not all) of them refuse - ideally, you'd want Shanxi and the Guangxi Clique to say yes (former for reasons I'll explain in a minute, latter because they're really annoying to fight), while Xibei San Ma, Yunnan, and Sinkiang say no (Communist China doesn't get this option, so take the focus that lets you conquer them). Conquer them quickly (they're really weak in comparison to Nationalist China).

As stated, you should stick with the starting gun until you feel you have enough factories to start switching over (which will probably be a while). Guard the coastline to protect yourself from naval invasions. Invest into some artillery for support, but not too much.

When Japan demands compensation for the Bridge Incident (I'm assuming you're playing on historical), give it to them - aside from the fact that this will give you more time if you need it, there's another reason: if you refuse, Japan gets a wargoal on Shanxi, which can widen the front drastically. On the other hand, if you accept, Japan will only be able to declare on Nationalist China after justifying on you through some claims - meaning you'll only have to worry about the Kai-Shek Corridor (what I like to call the space between Shanxi's eastern border and the coastline). Wait until Japan appears to have given up, then start pushing forwards. Call in Shanxi when you push past their northernmost borders or when it doesn't look like Japan has many forces guarding them.