Sometimes I want to vote on important questions, but I do not get any link or option to do it.
Is there any rule for voting?
The features you have available to you depend on how much the system trusts you. This is determined by reputation.
Concerning voting specifically, you cannot vote up until reaching 15 reputation, and cannot vote down until reaching 100 reputation. There is a complete list of privileges and reputation votes required for each of them on our FAQ page.