To use WLCG you must possess a personal digital certificate from a Certification Authority (CA) recognised by WLCG. The certificate will be needed both during the registration process and also when you submit jobs to the grid.
See https://www.racf.bnl.gov/docs/howto/grid/pemtopkcs12 for converting and importing certificates on various browsers and their versions.
Important: The private key associated with your certificate should:
Failure to observe the above rules could result in you being denied access to WLCG