You could be facing some jail time for this violation. You allegedly mistaking the community service as an option is not going to be a valid excuse. The court allowed you to pay off your fines with community service; this was not optional, it was mandated. You are entitled to an attorney for a probation revocation proceeding. Since you cannot afford one, request a public defender immediately.
If you are not in jail, I suggest you also try get as much community service done as possible between now and your court date. You are in violation, but if this was truely a mistake, then make up for it by fixing the problem so you can show the court that you are trying to comply. Also, make contact with the local public defender's office so that you can have counsel for this court date.
When people are given probation, the sentence will carry "general" conditions of probation and sometimes some "special" conditions as well. Payment of fines and fees are common general conditions. When you have violated a general condition, the judge can place you in jail for the duration of your probated sentence, but if the sentence exceeds two years, the judge cannot place you in jail to exceed two years. Since you have a misdemeanor sentence, you are looking at the maximum sentence in jail being the balance of your sentence. Now, you might have avoided this by fostering a good, open line of communication with your probation officer. That officer might have exercised some discretion and worked with you. At this point, that ship may have sailed. When you see the judge, you will want to try to show some proof that you have looked for work. You will want to be very apologetic about the misunderstanding. The judge has total discretion to hammer you, simply scold you or forgive you. An attorney at your side could help, particularly if the attorney knows the judge. However, it may also not be effective. Best of luck.