Establishing New Credit

[Justin True]

Hey first time poster here. Was hoping for some advice or tips from anyone who understands the credit industry or went through something similar.

PROBLEM: So this weekend my wife and I went to meet a couple lenders to get a pre approval letter so we could purchase our first Multi Family. We have our down payment set and her credit is in the 700's but the problem is I apparently have no credit. The lenders said they can only use my income in loan amount figuring if I have a credit score, so in order to take out a loan the size we were looking I need to establish credit. With rates at an all time low I'm hoping to establish some credit asap to take advantage.

ACTIONS TAKEN: I applied and was approved for a Capital One card and a Kholes store credit card. I also explained the situation to my parents and my Step Father who has very good credit said he would add my name to one of his cards he has, apparently that'll make his score help mine.I dont really know how all that works.

REMAINING QUESTIONS: Are my actions the right ones? Are there other good ways to build credit fast? Online it says it may take 6 months to establish a FICO score I'd hope I could cut that in half at least. 

TLDR - I have no credit and need some to get a mortgage. What are the fastest ways to get good credit???

Thanks in advance!!!