1) go to http://blast.ncbi.nlm.nih.gov/Blast.cgi-In that site you can see BASIC BLAST
2) among that you have to select one option for eg:nucleotide blast (The input sequence should be an nucleotide seq) "click it"
3)You will see a page titled as "Standard Nucleotide BLAST" a text box will be there with caption "Enter accession number(s), gi(s), or FASTA sequence(s)" paste your sequence* in that box.
4) in the bottom of the page you will see a button BLAST "click it" it would take some time then it would show the similar sequence .
sequence*-Download your desired seq in FASTA format from NCBI
BLAST tools requires to know 1) basic PCR and its applications, 2) what is the aim of your search, 3) how to use program softwares related and its applications inside genetic engerining , please identify your field required and scope of your search