Please find all information about your visa on the official webpage of the Federal Office for Migration or contact your embassy if you do not have experience in this matter.

https://www.sem.admin.ch/sem/en/home/themen/einreise/merkblatt_einreise.html

Official invitation letters in order to apply for a visa can be ordered after paid registration at the WIRM Assistant: wirminfo(at)wirm.ch.