Both Fasta and Genbank formats are text files, not compressed files. By text files, I mean that any software similar to Notepad can open fasta and genbank file formats. Fasta and genbank formats are different ways of representing a nucleotide sequence. Fasta format generally contains the sequence ID and sequence, while genbank format contains not only the sequence ID and the sequence but additional information such as definition, accession, version, keywords, source, features, etc. To learn more about these formats, I suggest finding any gene of interest in NCBI gene/nucleotide database, for e.g., GM2A gene https://www.ncbi.nlm.nih.gov/nuccore/NG_009059.1?report=genbank&log$=seqview&from=5134&to=22389 . At the above link, you will see the GenBank format of the GM2A gene. On the left corner of this page, just above the gene name, you can switch between different formats. On the right corner, you can send to ‘file’ as GenBank, fasta or other formats. Once you download these files, you can also try opening the downloaded GenBank or fasta files in software such as notepad.
If you need an easy way to convert your fasta format DNA sequence. You can use software such as ‘Snapgene’ (free version). In Snapgene, you can create a new DNA sequence and save it as GenBank format. This genbank format will probably be accepted by the program you are trying to use. However, it will not have all the feature annotations of your DNA sequence. Although it is possible to add the features manually.
Hope the above description will help you in getting started with the goal you want to accomplish. All the best.