Unfortunately in order to fly to BCN on UA you’ll have to go through EWR. UA has a nonstop EWR-BCN for the dates you mentioned. If that is what you want to do I would start looking for PZ fares, like WineCountry mentioned, on the EWR-BCN-EWR legs. Not impossible as a non elite, but you’ll be at the bottom of the waitlist, but there is still a chance that upgrade space opens before flight day when the waitlist is more sparse.
All of the topics listed by JALsnipe would help you immensely. You’ll learn from the best on here.