what will happen if my subscription will expire? will i be able to go back to downloading apps without them asking to pay for the app i did not want to pay for?

The prompt I am getting is “ there is a billing problem with a previous purchase.” And I guess it won’t let me download any apps until I pay for that app?

If that is the message that you are getting, and assuming that is the app that you owe iTunes for, then yes, you won't be able to download anything else until it's paid.

I try putting in my card but it says declined. I’d it because i took the money out of it?

Not sure what the second part of that is supposed to mean, but if the card is declined you are unlikely to be able to use it with iTunes, unless its' the card issuer declining it for iTunes, or there is an error which you can correct (e.g. if the card's registered address doesn't match what's on the iTunes account). If iTunes is attempting to take money from the card you might see : About payment card authorisation holds in the iTunes Store - Apple Support

can i get out of that subscription and never have to pay for that app again once I pay for the bill I guess

You can stop a subscription from auto-renewing : View, change, or cancel your subscriptions - Apple Support

What is the text of the message/prompt that you are getting, why do you think it's related to the subscription ? Why do you think that you are being asked to pay for something ? Obviously when you have payment details on an account they have to be valid details and continue to pass the iTunes validation.

what will happen if my subcription expires?

If your payment details are no longer being accepted by iTunes a subscription expiring is extremely unlikely to make any difference.