Well, SEO is a very wide area. You need to be specific. There are many aspects of an SEO that you may need to improve in your site. For instance:
- On-gage SEO
- Off-page SEO
- Technical SEO
Each of these will use different tools. But the tool that you easily understand is what you should go for. Plus, it also depends on your budget. Some very good tools like Ahref are very expensive. You can use Ubersuggest or Keyword Everywhere which have browser extensions that will help you get the right keywords for your website.
I use a combination of 5 tools: Ahref + Ubersuggest + Yoast SEO (wp plugin) + Long Tail KW tool + Keywordshitter.
Yoast SEO will help you with on-page SEO, finding the pages to link withing your site. The other tools are good for competitor research and off-page SEO (link building).
This is a very broad question you have asked. If you need more info on any specific tool or assistance, I will be willing to help/answere. I am also learning, so I will learn better by teaching you.