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

Farm solutions aren't supported in SharePoint Online, and so any existing solutions will need to be rebuilt using supported tools. What that exact path is will be determined by what components are in the solution. For example, instead of server side web parts, we now have App Parts, SPFx, or PowerApps, Instead of timer jobs, we can now use Flow, PowerShell, or Azure Functions. Instead of event receivers, we now have webhooks, etc. The new tools are compelling, but the transition is not simple or trivial. As the others have noted, there's not enough info to provide specific guidance, so feel free to add details about what specifically the solution is doing. Or, check out the migration docs, as noted in another post, and check out the dev documention: https://developer.microsoft.com/en-us/SharePoint/