I recently started writing novels and I wanted to know where is the best place to post it as I want to have some opinions on my novel so that I can improve.
That's really vague! What is your novel, exactly? What is the target audience? You have several options:
Archive of our Own, which has a lot of stuff, but people mostly go on their for fanfics
Wattpad, which has a lot of original stuff, but it's not exactly known for quality
Specialist art sites like Deviantart or Furaffinity, depending on what your target market is
Your own website/blog/tumblr, but it's going to be harder to have people discover your work
If you want actual constructive feedback, though, writing sites like this are the best. If you post your first chapter, we can give you feedback and suggest ways to improve it, which probably won't happen on other kinds of sites. The feedback you get for the start of your story generally also applies to the rest of the story, too, so you can take that and get to revisions to make your entire work stronger.