For understanding SEO you will need the following tools:
1) ahrefs - it's an amazing tool to find the right keywords, and backlink profile. Find incorrect or spammy links and remove them using this tool. Ahrefs is expensive, so I'll suggest a way to get it for a cheaper price, below.
2) smallseo.tools is a good website having all the basic tools in one place. It has all the cheap and simple tools to save your time and costs nothing.
3) You will need keyword analysis. (Which keyword to rank for). This you can do using the following steps:
-install keywordseverywhere chrome extension in your browser to get that data.
-mozbar is also and additional free tool you can try, which will tell you whether you can rank your site for that keyword or not. But it's messy you would only want to activate it when you're working.
-in moz, check the DA, PA and spam score of the domain you are doing the SEO of. Check spammy backlinks and remove them manually.
4) Keywords.io is a good tool for checking which keywords you would want to rank for. This is also expensive like ahrefs. But I'll give the trick below to get both of them at a cheap price.
5) Install plugins to increase sharing of content from your website. This increases social signals that helps you get more SEO.
6) After you have selected the right keyword, have the right social signals you will want a high DA, PA for your appropriate ranking. Ask someone with a domain that is high in DA, PA to give you some backlinks for free (ask those who are in your personal network. You can ask me for some backlinks also and I'll try to provide them to your for a cheap price, even for free if you need just a few, in the case we end up becoming friends after the call from domains that have DA, PA greater than 20, for example from lekobaza.com)
7) If you haven't already bought a domain for your business, you can ask me. I'll try to provide you with the best domain for your business for almost no cost because I really love helping startups. It's in the long run that if you grow and like my service you will pay me more automatically.
8) Use flikover.com, to guy bulk seo tooks for free. There are other services as well. You will be able to get ahrefs and keyword tool for a cheap price.
9) Write a lot of blogs from your own domain. These blogs also increase your authority, and also give traffic. Make sure whatever you are writing is beyond 1000 words, because you get a thin content penalty from google and write great content because that way you will be able to get more social signals as well as a lower bounce rate - both of which will increase your DA, PA and get you automatic SEO.
10) Guest post for high authority sites and in your guest post, include backlinks to your domain.
11) Link all your social media sites to the same domain and make it easy for people to find you.
12) Answer a lot of questions relevant to your niche on quora and ethically, if it's not reducing the quality of answers on quora, in a disclaimer, include a backlink to your domain. Backlinks from Quora are really helpful.
13) Reddit is another way to do great SEO but that's hard. Since you're doing a startup, I will not advice you to do that right now, because that will take time to master and you don't want to be wasting time there.
All the best !