Start with the organism search in the NCBI Assembly database. For example:
(If you do not find an Assembly record for your organism, read more about finding a genome.)
Select the assembly that you want, for example the Representative assembly that you can access by filtering your results on the left side of the screen on the search results page.
Once you access an individual record (example) use the Nucleotide links under Related Information on the right side of the screen to display either the GenBank/INSDC (submitted) records or RefSeq (NCBI- derived) Nucleotide sequence records belonging to the selected assembly.