You are attempting two steps at once, from Windows to Linux, and from Delphi to Lazarus. I guess you will stumble and fall. Do steps one by one.
First convert your project from Delphi to Lazarus, while staying on Windows. Your only task probably is to add the directive {$mode delphi} to your project units. You may have problems with strings: In lazarus a string consists of one-byte entities, in Delphi it consists of two-byte entities; normally this is transparent, but some low-level and system calls can fail. You'll have to convert these strings to WideString, the chars to widechar, and the PChar to PWideChar, plus some issues with UTF8-conversion - all these are solvable.
When you succeeded with the migration you can proceed with the Windows to Linux conversion. Here you must replace all usages of the windows unit by LCLIntf and LCLType, and Messages by LMessages (or simply remove Windows and Messages from the uses clause which are deliberately added by Delphi). If you have windows messages in your code, replace the WM_* messages by LM_Messages. If you are lucky this will be enough. If you are not lucky ask the related questions here. In the worst case, the project will not be portable without major rework.