first, you should specify the species and find the most conserved region by the help of alignment (you can use Ugene software) then design the preferred primer with the target size you need by prime3 tool in ugene software

Please specify the details of the species and the gene also your requirements. The common method is downloading the FASTA sequence from NCBI of the gene and use Primer 3 to pick up the primers.

You can always use Primer BLAST for recommended ones. How variable is the genome of your species at that locus? If there are too many SNPs within the population you can find the reference genome is not always accurate. What is the GC content of your amplicon?

thank u for your replying but these techniques are very hard to me to use it