How do you make a program for a website? What type of skills/code should I know?

I'm trying to make a mortgage calculator for my website. I want to create it myself and not use another person's code. I want to create a form where someone can fill in the following fields:

1. Sales price of home (In Dollars)
2. Percentage down (%)
3. Length of Mortgage (In Years)
4. Annual Interest Rate (%)

This will then do computations it needs to do to give them all the information they need.

I would like the form to give the visitor details like this example I made:

Mortgage Payment Information

Down Payment: $15,000.00

Amount Financed: $135,000.00

Monthly Payment: $898.16 (Principal & Interest ONLY) Since you are putting LESS than 20% down, you will need to pay PMI (Private Mortgage Insurance), which tends to be about $55 per month for every $100,000 financed (until you have paid off 20% of your loan). This could add $74.25 to your monthly payment.

Monthly Payment: $972.41 (Principal & Interest, and PMI)

Residential (or Property) Taxes are a little harder to figure out... the average residential tax rate seems to be around $14 per year for every $1,000 of your property's assessed value. Let's say that your property's assessed value is 85% of what you actually paid for it - $127,500.00. This would mean that your yearly residential taxes will be around $1,785.00. This could add $148.75 to your monthly payment.

TOTAL Monthly Payment: $1,121.16 (including PMI and Residential Tax)

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I have knowledge using C++, HTML, and CSS. What resources or skills should I have to execute this type of project. I also have a Amazon Web Service (AWS) account.

Any information provided will greatly help me.