how much will a secured credit card help my score?

just found a citi mastercard that has a minimum of 200 purchase apr is 18%. then it said this. i wont summarize i copied and pasted it. “Build Your Credit – and Earn a Competitive Interest Rate1 Deposit an amount, from $ 200 to $ 5,000 into a CD (Certificate of Deposit). Your credit card limit then becomes equal to the amount of money in your CD. We report your account performance to major credit bureaus, and after 18 months, you could become eligible for an unsecured Citi® Platinum Select® MasterCard®. Meanwhile, you’ll earn a competitive annual interest rate on your CD.”

if i was to for the next 18months keep this card(if i get it) in good standing how much will it actually help my score.
next month I am also paying off and closing a defaulted account that has been after me for a couple of years

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