I received a summons for 2C:33-4a for harrassment through text...If found guilty what penalty am I facing...first offense.

This is a petty disorderly persons offense for which if convicted the worst case scenario could be up to 30 days in jail. Though I don't believe as a practical matter that jail time would be imposed I suggest you consult with an attorney. S/he would best be able to advise and guide you.

They will probably have to show much more than just one text, unless you are clearly harassing them or threatening them.

More information is needed to fully understand your question. However, Harassment is a petty disorderly person's offense, the lowest criminal charge in the 2C Code. On a first offense it is highly unlikely that there will be anything but a fine (max. $500) although probation and 30 days county jail are permitted (so unlikely I would not base decisions on the possibility). The real issue is your record for future employment. The arrest is likely already on the CCH and that and a conviction will stay on for 5 years. There is no need for an attorney on the potential sentence but you may consider an experienced criminal defense attorney to fight the case because of the effect on your record. Call with more information if you like.