Unless the state in which your aunt lived has a "small estates" procedure and your aunt's estate would be small enough to qualify to use it, your mother will need to file a petition to be appointed as the administrator of your aunt's estate with the probate court (or Surrogate's Court) in the county where your aunt lived.
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.
You need to contact the Probate Court of the county in which you aunt lived. There also needs to be an administrator appointed for your aunt's estate. Your mother would be a likely candidate to be an administratrix for the estate. The Probate Court will be able to provide documents to you for the application for appointment as administratrix of the estate. Google "probate court" + the county where your aunt died to get contact information for that county's probate court.
This answer is not intended to provide you with specific legal advice regarding your situation, or to create any attorney-client relationship.
My condolences to you and your mother on your loss. The first step in dealing with an estate where there is no Will is generally to determine what assets the deceased person had and whether any of them need to be dealt with by an estate administration or if everything passes outside probate. In general, if the deceased person owned real estate, then it is likely that an estate does need to be opened. If she owned mainly financial assets such as bank accounts, savings bonds, IRAs, life insurance, etc., then it is possible that few or no assets actually pass into the estate. If an asset such as an IRA or life insurance exists, then that asset will generally be distributed directly to the person(s) stated in the beneficiary designation, although if no beneficiary was provided those assets could also end up in the estate. Regular bank accounts and other financial assets normally go into the estate, unless they were owned jointly with someone else or have a "payable on death" or "transfer on death" designation.
If your aunt lived in Georgia, and if it looks like there will be some assets in her estate, then your mother and any other heirs have a couple of options. One option is for someone to file a Petition for Letters of Administration in the appropriate county probate court. www.gaprobate.org is a helpful website for Georgia probate information and a Petition for Letters of Administration can be downloaded from there. I would urge your mother (or whoever will serve as the administrator) to consider consulting an experienced attorney for help in filing the Petition. One thing to keep in mind when filing is that it is possible to waive the requirements for bond, inventory, and reports if all the heirs agree to do so, and this request should ideally be made on the original Petition. Having those items waived can make the process easier and less expensive and time consuming. In addition, the attorney can help the administrator correctly handle all the other estate administration tasks.
The other option is for the heirs to agree on a division of the estate but to decide to file a Petition for Order Declaring No Administration Necessary. This option is only available for estates where the deceased left no Will. In very small estates without a lot of debt and with heirs who all agree on how to deal with the property, this option can be easier than a full-blown administration. However, filling out the Petition for Order Declaring No Administration Necessary requires a lot more specific information about the decedent's assets than filling out a Petition for Letters of Administration, and it may not always be possible to obtain that information without having an administrator appointed. Again, the help of an experienced attorney can be invaluable, but www.gaprobate.org is a good source of information for people who want to research these issues on their own.