What types of credit is needed to get a high score?

My credit score is about 615. I am working on improving my score so that I can qualify for a mortgage loan with low financing in about a year or so. I currently have an auto loan, and 3 revolving credit accounts. (All bank credit cards) Two of the revolving accounts are new, and the other almost a year old. My auto loan will be a year old in July of this year. Should I apply for more credit to get my score higher? If so what type of credit? What is a good mixture of credit to have to get a person’s score higher? Any advice is appreciated. Thanks!

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