If this is the only card you have then use it sparingly. Your card is being reported monthly, whether you're using it or not. In some instances, banks will close accounts after six months of inactivity, so every once in a while you buy something small and immediately pay it off just to keep the account active.
Paying a $119 cable bill with a card that has a $700 limit puts you at nearly 20% utilization with nothing else being charged. High utilization (anything north of about 20-25%) can put a small temporary drag on your credit score, so if you're planning on applying for additional credit, keep that in mind.
Use one of the free services that lets you follow your credit score for free (such as Credit Karma, Credit Sesame, or NerdWallet), because they do provide some useful advice and recommendations for other credit sources based on your credit profile. Besides, you need to stay informed at all times where you stand as you build your credit profile.
Paying a cable bill that isn't in your name with a credit card that IS in your name doesn't do anything to build your credit. You're probably thinking of those commercials that talk about boosting your credit score using subscriptions or other bills you already pay. The fact is, those programs only apply to the credit score of the bureau that offers them. In other words, the Experian Boost program has NO EFFECT on your TransUnion or Equifax scores - it only impacts your Experian score, and it takes a certain number of accounts being paid for a minimum period of time before there's any effect at all. Paying a cable bill alone won't do anything.
You're 18 and already have a credit card - good for you! (smile) But treat it like gold - don't go crazy with it (sounds like you're being smart so far), and don't go running around applying for every credit card in sight either. Take your time, build a careful portfolio, and use your credit carefully and only when you can justify it.