To find out what sequence data are available for the organism of your interest, start on the NCBI home page.

Select the Taxonomy database from the All Databases search menu, enter the organism name in the Genus species format (for example Mus musculus), and perform your search. (If you do not retrieve a record —after you have assured that the spelling is correct— it means that there are no sequence data for this organism.)
On the Taxonomy search results page, click on the organism's name which may open up an additional listing of organisms at the subspecies level.  Select the organism that you want and click on the title to open up its record. Check the Entrez records table (on the right side of the screen) to see what databases contain records for this organism. The number of databases and the number of records can vary markedly between organisms.

The Assembly database will be listed in the table if there is a genomic assembly for the organism. The number of Assembly links indicates the number of individual genomic assemblies for the organism. Clicking the number will take you to the Assembly database records that contain metadata for assemblies. From the Assembly database you will be able to access the sequence records (In the Nucleotide database) and download the data for one or more assemblies.

If you want to access information for all genomes/organisms at the species level, follow the Genome link in the Taxonomy Entrez records table.
If you find neither the Assembly nor the Genome database in the Taxonomy table, check for links to the BioProject database. In the BioProject database you will find all publicly registered large biological projects for the organism. Once in BioProject, use the facets on the left side of the screen to select the Genome sequencing Data Type if available. Within an individual project, you will be able to check for any type of data available for the project.