It's had. "You had better clean your room" becomes "you'd better clean your room!" "You had better do your homework" become "you'd better do your homework!"

As Virginia said, the 'd' is the contraction of 'had'. Note that, unlike in Spanish, the word 'advice' is uncountable. We cannot say 'an advice' and we cannot say 'advices'. We 'give advice'. If you really want to specify the number of 'tips', you can say 'a piece of advice' or 'some pieces of advice', but it isn't usually necessary. We'd usually say 'Can you give me some advice?' or 'Can I give you some advice?' Note also, that 'You'd better' isn't really a suggestion. It's a very strong piece of advice, or even a command.