@Intissar ********
Tunisians apply for visa through the Royal Thai Embassy in Rabat, Marrocco.
You should visit their webpage to read more about their requirements and processing time. There are no such thing as "get it immediately". You apply online at thaievisa.go.th
After you sent your application with the required documents, they can still get back to you, to ask for additional information as well as request an interview (I've seen cases where the applicant was located far from the embassy and the interview were done by videocall).
Once your visa is appoved a pdf-file will be sent to you by e-mail. You print the visa and show it together with your passport when you travel to Thailand.