Q: How do I spent my store creidt

You need to redeem the Gift card onto your account if you have not done so.

Redeem iTunes Gift Cards and codes - Apple Support

Once its redeemed. Its balance will be used when purchasing items form the store, before the associated CC is.

How iTunes Store purchases are billed - Apple Support

Chunshi,

Store credit can only be spent on purchases.  It cannot be spent on gifts.