How is a specific fragment isolated for PCR amplification?
There are a number of primer testing tools that will compare primer sequences against the genome to ensure specificity, or help you design specific primers to amplify a specific region. One place to start is NCBI Primer BLAST; the linked page explains how to use it to generate primers to amplify a specific gene.

Doing PCR on genomic DNA can be a bit tricky, though, since you have to deal with a lot of confounding factors like DNA accessibility, contamination, and PCR inhibitors that get through your purification procedure. In addition, genes often contain huge introns, so you're going to end up doing PCR on an extremely long template (which is very hard or even impossible, depending on the size of the gene) and you'll end up with the whole gene sequence with introns which is not what you need in order to express it as a transgene.

Instead, I'd recommend purchasing a cDNA clone of your gene of interest and amplifying it from that. cDNA is made from the spliced mRNA sequence, so it will be a much more managable length and will prevent problems with alternate splicing.

If you go to the NCBI page for a gene, on the right sidebar under "Related Information" you'll see a link to Order cDNA clone. This will give you a few options to purchase a purified plasmid containing that gene for around \$60 - \$90. It will be much easier to work with this plasmid than genomic DNA and it will definitely save you time and money, especially if you're still inexperienced at molecular cloning.



Of course, it's not 2003 anymore and there's a good argument to be made that the whole idea of doing molecular cloning yourself is a waste of time and money. If your budget allows it, the easiest and fastest way is to pay a company to synthesize your gene for you. Genewiz, for example, offers snythesis for $0.28/bp, but you can probably get a better deal if you shop around.

The advantage of synthesis is that you can get the exact sequence you want cloned into exactly the right vector done perfectly the first time. If you don't have experience with it, trying to clone a gene yourself will take longer and cost more money than you think. It's better to just get it synthesized so you can move onto your real experiment.