What is the best practices to migrate SharePoint farm solution to SharePoint online?

That’s a huge question to answer in one go here! Suggest you breakdown the problem into manageable chunks. Is your org already on Office 365? Do you plan to be all in to the cloud or hybrid? What version of SharePoint are you using today? Microsoft provides some assistance with Fast Track but depending on complexity you may need a partner to support you. Not enough information here to really offer assistance more than this. Start by looking at the official Microsoft resources. Remember going to SharePoint Online involves Office 365! Good luck.