just signed up for a costco american express card ????

so i have been trying to raise my credit so i can finance a motorcycle i was told that if i opened up an amx car through costco it would actually raise my score a smidge if i was approved, so i was approved but is this true? i always thought when u opened up an account it lowered it ,i plan on using this card every month mainly for gas and what not and have it payed off on time but i was just curious if this is a good move to build credit sooner, ps i already have an auto loan with my dad as a co signer, thanks !