Building credit is easy. The key is to not go crazy with opening up a bunch of credit cards, but rather be smart about what accounts you open. Have a plan for your charges, making on time payments, and paying the balance off within a quick amount of time to avoid costly finance charges. Consider purchases that you would already be making, but instead of paying in cash, put them on your new card, and put your cash on the card to pay it off. You might even consider choosing a card that has a promotional starting interest rate of 0% or some small percentage. This way, you can let your balance ride for a few months while you make timely payments and establish good payment history before paying off the balance before the promotional interest rate drops off.
Don't wrack up your card with unintended purchases, or impulse purchases. The line of credit is a business tool for you to use on your investing journey.
Also, make sure you diversify your types of credit. Don't open a bunch of credit cards. A credit profile that has a credit card, a car payment and a personal loan (and of course Real Estate) would make a good mix for creditors to consider, and the credit bureaus to evaluate for a score. Again, make sure you have a plan for paying it off. Don't extend yourself beyond what you can pay back if required.
There are types of alternate credit that can be used. Payment history on your utility bills, cell phone, and other types of bills can be used to establish credit. Go to the credit bureau sites and investigate what they will consider to give you a credit score.
FHA will be your go-to lender for starting out (beside Hard Money Lenders with higher interest rates). Your first deal might need to be a house hack in order to use the FHA loan, but you will be surprised how quickly a year goes by, which is the FHA requirement for occupying the home.
Lastly, if you find yourself having trouble getting anyone to give you a credit card or loan, consider a pre-paid credit card. These allow you to place a deposit on the card, and have a credit limit equal to that deposit. These still report to the credit bureaus for payments made, time they are open, and usage history.