I recently got pulled over for expired tag and the police smelled alcohol so he breathalyzed me and I blew a .029 underaged. What could happen to me?

You certainly could have been charged with DUI.  In Georgia, a blood alcohol level of 0.02 grams or more is sufficient for DUI in an offender who is under 21.  Under Georgia Law, OCGA § 3-3-23, it is illegal for someone under 21 to even possess alcohol. Further, the courts have interpreted this law very broadly and, as a result, "internal possession" of alcohol by someone under 21 can be a crime.  But, generally speaking, a standard under 21 alcohol charge will not result in a license suspension.  Keep in mind though, the State's prosecuting attorney is not limited to what the officer wrote the ticket for - he/she can charge you with any crime that the facts and circumstances support. Under Georgia's underage drinking laws, it is not necessary for the defendant to have an alcoholic drink in hand to be convicted of Under 21 Alcohol Possession.  The defendant having alcohol in his/ her blood or on their breath can be sufficient enough to be found guilty.  Even without alcohol being physically present, law enforcement will attempt to determine whether the defendant has any manifestations of consumption (odor of alcohol, blood shot eyes, slurred speech, coordination issues, etc.).  Based on the limited facts provided, it sounds like the officer has at least this much.  Often in underage drinking cases there is an admission of consumption by the defendant, and / or a positive reading on a portable breath test (PBT) like there is in your case.  Due to the pervasiveness of Under 21 Alcohol cases, the Georgia Legislature has given Georgia prosecuting attorneys some options in disposing of these cases.  Most common are the "pretrial diversion" and the "conditional discharge."  In either scenario, the defendant is allowed to plead guilty but later have the charges dismissed if certain conditions or terms are met.  Generally, the terms are probation, a fine, completing an alcohol awareness class, performing community service and other terms and conditions.   If a defendant neither admits guilt nor declares innocence he is entering what is referred to as a Nolo Contendere plea, or Nolo.  By entering a plea of Nolo, the defendant agrees that the court may consider him guilty for the purpose of finding him guilty and sentencing him.  Often, an under 21 year old in Georgia may be misinformed and not realize that entering a plea of Nolo has basically the same legal effect and consequences as being convicted!  Further, as a conviction for an Under 21 Alcohol offense may affect the defendant's school enrollment (many schools have an alcohol policy) and scholarship/financial aid eligibility, I would recommend against a simple Nolo plea and going to court without an attorney.  A competent attorney can help protect you against all the potential ramifications.          

Most Georgia jurisdictions have a diversion program.  You should speak with an attorney to discuss your options.