Plug your competitor's URL in SEMrush and copy the complete list of keywords. Search those keywords in Google and analyze the results on the first page and see whether you can rank for those or not.
If top authority website of DA 90+ are ranking on the first page while suppose the DA of your site is in 30's, it's obvious you won't be able to rank for that keyword unless you use a PBN or any other blackhat technique which is never recommended.
Repeat the process for all the keywords and pick out the best keywords.
Plug those keywords in Google keyword planner and know the rough search estimates for those keywords.