For the app as a product, it would be international class 009 and the description would likely be
"Software to enable uploading, posting, showing, displaying, tagging, blogging, sharing or otherwise providing electronic media or information over the Internet or other communications network." [the description used by Facebook]. If you will be providing a service where they log on to your server then it is international class 042 and the description would be something like "Computer services, namely, hosting online web facilities for others for organizing and conducting online meetings, gatherings, and interactive discussions; and computer services in the nature of customized web pages featuring user-defined information, personal profiles and information" [the description used by Facebook]
You can see how Facebook did this by referring to their multi-class US Trademark Reg. No. 3948096 for a stylized "F".
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
My colleagues have answered your question as best they can given the information you provided. I would like to add that if funds are limited, they should be spent wisely. a trademark can be established by first use or by filing an application. It may be wiser to spend the money elsewhere. Many colleges have assistance programs for small business owners which you may find helpful.
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