The tools –
1.Google keyword planner
2. longtail pro or serpstat (they are cheaper than semrush but do the job well)

The process –
1.Search for all your keywords on Google keyword planner, ubersuggest, answerthepublic and quora, keywordtool (.io)
2. Paste all the keywords you have collected into longtail pro or serpstat to get their data.
3. Decide on which ones make sense to you in terms of relevancy to your product/service, cpc and search volume