So what is SEO?
SEO (search engine optimization) is the practice of optimizing your website and web pages to climb higher in Search Engine Results Pages (SERPs). The aim is to increase traffic by improving visibility as the first business users see when entering relevant keywords into a search engine. You’re targeting higher-quality traffic who are more likely to convert, as well as driving volumes of traffic to your site whilst raising brand awareness.

The 3 Types of Ecommerce SEO

Three main types of SEO apply to eCommerce websites: 
On-site
Technical
Off-site

On-site SEO refers to any actions you can take to boost your ranking within your domain. These include items such as product descriptions, product metadata, and headlines.
This part of page optimization can be achieved by the business itself if the business wants to save some money.
There is an App in Bigcommerce and Shopify which can help score the work you have done.

Take an example as below:

You will see how your page looks like in google by using Google meta description tester

Simply put the link and the green means the title and meta description are good.
The optimization of the products, the categories, pages, brands, and blogs are similar and it is tedious and time-consuming work.
 
Off-site SEO are actions you can take outside of your website to boost ranking, like link building and affiliate post creation.
Link building is the most difficult part in SEO, Link building is the practice of building one-way hyperlinks (also known as “backlinks”) to a website with the goal of improving search engine visibility. Google now focuses on link quality (not just link quantity). simply put, if you get more pages with high DA(which I have mentioned above) so-called quality links linking to you, your rank in Google will boost quickly. 
Luckily there are many apps to check the DA score, here is one

A new site or a site without doing any SEO normally has a lower score.

The SEO company who claims to rank your site in 30 days at a very low price, you’d better check their DA of their website first, because a real SEO company knows what they are doing and DA value won’t be too low.

When you hire someone to do guest blogging, the least of DA score should be 18+, the higher the better. The quality of links also depends on the correlation of backlinks. Otherwise, it’s very likely to be penalized by Google. With that said, links are not the more, the better. Always look for quality links and apply content marketing in your SEO strategy. High-quality content can return you with better links

Backlinks are important and content marketing is a way to get better links. Don’t let any so-called “SEO pro” to ruin your business.

If you want to hire someone to do link building, maybe start from your competitor’s backlinks, which you can download fromSEMrush, filter the nofollow links, error links, and get high quality related links for your link building campaign.

Technical SEO refers to the geekier aspects of SEO, such as 404 errors, hosting issues, duplicate URLs and so on. Audits can help you root out problems like these and eliminate them.  and ensure that your site can be crawled and indexed. 
Here are the most common areas of best-practice that you need to pay attention to.
Technical SEO Checklist
Make Sure You Are Using HTTPS
Fix Broken Internal and Outbound Links
Find and Fix Crawl Errors
Make Sure That Your Website is Mobile-Friendly
Add Structured Data
Add this app SeoQuake to your chrome for free, it helps you diagnose the technical problems of your site

Above, you will have a general idea about SEO, and pay more attention when you encounter some scams like the below:
Scam #1. “We Guarantee Page 1 Rankings”
Scam #2. “We Have the “Secret Sauce”
Scam #3. “You’ll Get Over 1,000 Visitors Per Day to Your Website!”
Scam #6. “We Do The Best Link Building”
Scam #9. The Free 30-Day Trial
You should never invest your time and money into anything that seems too good to be true, 

Here are some facts:
You can’t expect to outrank your competitors on a $250 a month SEO package, and you should never trust this common pitch or any pitches like it.
EVERYONE would be #1 if it was that easy to rank in google.
There is no short cut in SEO, it takes time. 
“targeted visitors” are nothing more than traffic auto-generated by a bot. Traffic by itself is just a vanity metric, your conversions will not increase as a result of this traffic.
The most difficult and important part is backlinking, and it cannot be done overnights or in a month.
SEO is just a part of your business and you cannot handle everything by yourself. To know the general knowledge of SEO can help you avoid these dramas and hire a real SEO pro
Here are more detailed suggestions in the video.