A friend told me that immediately after sec I should clean my vagina with some liquid to avoid chances of contracting sexually transmitted diseases, even if I have it with only one partner.
I want to know which liquid to use or what exactly to use and the detailed cleaning procedure.
Eeesh, don't do that. You won't avoid STDs that way, and can upset the entire ph balance of your vagina, which is self-cleaning. The best way to avoid STDs is to use condoms.