How to find the location of a gene, starting with the gene name and the DNA sequence of nucleotides?
To find the genomic location of a gene when you start with a gene name;

Use the UCSC Genome browser. This has a number of advantages. There are a number of species to choose from. It provides a representative image of the gene and regulatory elements that surround it. It can display a lot of extra information if you want to see it. The disadvantage ot the UCSC Genome browser is that the information density may overload non-professional users. 

To find the genomic location of a gene when you start with a DNA sequence;

Use the search tool that links to the UCSC browser. 

The alternative option is to search for your gene ID or sequence in NCBI Gene. 

To search for a DNA nucleotide sequence using the link above, select Reference genomic sequences (refseq_genomic) from the pull-down tab in the Database field. Limiting the search to a species using the Organism field is also wise.

Both of these sites provide the information you are after. Taking time to read the help instructions will be useful.