This is the exact opposite of mistake #1. Let me start by revealing a dirty little secret when it comes to website design. Most people think, Oh, I’m going to hire a professional website firm because I want a “real website” As opposed to a Word Press website or something else. And what most people don't realize is that all websites are made from the same stuff.
The ADA is a civil rights law that prohibits discrimination against individuals with disabilities in all areas of public life, including jobs, schools, transportation, and all public and private places that are open to the general public. The purpose of the law is to make sure that people with disabilities have the same rights and opportunities as everyone else. The ADA gives civil rights protections to individuals with disabilities similar to those provided to individuals on the basis of race, color, sex, national origin, age, and religion. It guarantees equal opportunity for individuals with disabilities in public accommodations, employment, transportation, state and local government services, and telecommunications.

Aloha Tatiana! I wish your questions had straightforward easy answers – and if you are government funded it kind of is. (See here: if you are not government funded, this is something that is currently being debated in the legal system. That being said, the WCAG 2.0 guidelines are a good set of guidelines to help protect yourself if you feel you should. (You can see more on WCAG here:
What services they offer. Make sure the web design agency you’re interested in offers all the web design services you’re looking for — like 508 compliance, ecommerce design, or CMS integration. You’ll also want to see if the agency provides other services like SEO and PPC. You’ll likely be interested in these services after your website is up and running, and it’s extremely beneficial to work with a web design agency that does it all. You’ll want to be sure that the agency you choose provides other services like SEO and PPC since a great website is essentially useless without the proper promotion.

I should mention one caveat to all of this. Businesses that are required to comply but don't have the ability to bring their websites into compliance can provide an accessible alternative to provide the same information, goods, and services that they provide online, like a staffed phone line. The trick, however, is that this option has to provide at least equal access, including in terms of hours of operation. And, as we know, the internet is around 24/7, so good luck with that. 
Title II applies to public transportation provided by public entities through regulations by the U.S. Department of Transportation. It includes the National Railroad Passenger Corporation (Amtrak), along with all other commuter authorities. This section requires the provision of paratransit services by public entities that provide fixed-route services. ADA also sets minimum requirements for space layout in order to facilitate wheelchair securement on public transport.[20]

Signed in 1990 at a time that most people hadn’t even used the Internet, the Americans with Disabilities Act does not explicitly regulate how websites need to follow nondiscrimination requirements. We now know that using the Internet is one of the most important ways for people with disabilities to fulfill their needs and desires. For many people with disabilities, especially impairments to sight and motion, visiting a store or other physical location can be a challenging experience. Online shopping, for example, allows people with disabilities to make the purchases they need easily and securely within the comfort of their own homes.
Many people with disabilities use “assistive technology” to enable them to use computers and access the Internet. Blind people who cannot see computer monitors may use screen readers – devices that speak the text that would normally appear on a monitor. People who have difficulty using a computer mouse can use voice recognition software to control their computers with verbal commands. People with other types of disabilities may use still other kinds of assistive technology. New and innovative assistive technologies are being introduced every day.
In 2001, for men of all working ages and women under 40, Current Population Survey data showed a sharp drop in the employment of disabled workers, leading at least two economists to attribute the cause to the Act.[52] By contrast, a study in 2003 found that while the Act may have led to short term reactions by employers, in the long term, there were either positive or neutral consequences for wages and employment.[53] In 2005 the rate of employment among disabled people increased to 45% of the population of disabled people.[54]
Recent years have seen an uptick in federal lawsuits filed against businesses and governments, alleging that their website violates the ADA by being insufficiently accessible to people with disabilities. In 2017, there were at least 814 such lawsuits against organizations in a variety of industries, from banks and credit unions to restaurants and e-commerce websites. The defendants include small businesses as well as major corporations such as Nike, Burger King, and the Hershey Company.