Not simple questions to answer and to fully answer them requires more than a PRE post, but to summarise:
1. Freely competing protection agencies that provide a set of laws in which you will be expected to comply if you are to receive their protection services
2. Through agreeing to contracts (this involves actual agreement not through some invisible social contract people pretend we currently sign)
3. They will provide these services for a fee
4. The same place we get money to pay for our existing monopoly services
5. You can choose to move to a different protection agency if you are unsatisfied with their performance or set of laws
For a more in-depth look at the issue:
https://mises.org/library/idea-private-law-society