They seem like pretty straightforward questions, so you should be able to find those answers in your textbook...
But anyway:
1. International laws are set out by treaties. Said treaties are proposed, written and set by international organisations such as the UN. They can only apply to the signatories of those treaties, and even then, very little can be done if an international law is violated. An international law is only valid within a sovereign state if the law has been ratified into domestic law by the government of that country. Sources of domestic law are usually state/federal governments and to a small extent, court decisions.
2. The legal system has to keep up with the changing values of society, otherwise it becomes redundant. Major influences include: changing social values, attitudes and opinions, advancements in technology and current world/domestic issues.
Examples of each influence:
Changing social attitudes: The rise of multiculturalism has lead to the introduction of new anti-discrimination laws
Advancements in technology: New types of crime have come up as a consequence of the widespread use of computers. Invasion of privacy, fraud and harassment/discrimination are examples of crimes that are now much easier to commit via computers/technology. Laws must be able to deal with such crimes.
Current issues: A very recent example is the proposal of an emissions trading scheme in response to global warming. Other laws have been recently been brought in to help protect the environment by setting rules and regulations. Current issues can be related back to the first two points.
3. It's up to you to determine the effectiveness of the legal system in dealing with those issues. Read case studies, look up the relevant laws and form your own opinions. It usually doesn't matter whether you agree or disagree with the question, your marks are mostly based on how well you make your case - articulation of arguments, relevance of case studies/laws chosen, etc.
Edit: These are only very brief answers. Make sure you look up these questions in either your textbook or class notes - more detailed answers should be there.