[Help] Question about how to connect two website through API

[syser]

First , I am a relatively junior user , only can install Joomla and install template , setting the website with ready-made argument .
I know that, this demand is a challenge to me ， I want connect two website through API , I want to know how should I do under Joomla , I know that , I need to study many things , but I am ready for this time .
Now , let me write down the steps , I think which need:
A: Joomla website B: Other website
1. B will have unique user id ，and A on the correspondence , one to one . through API
2. B will be passed to the A lot of resources ， through API
3. The users of A will doen the task(resources from B), and the results will be passed back to the B , it will record the unique user id of B(which corresponding account to A)

I want to know , how should I do with my websites , where should I do the settings ? Or I need create some new scripts？

[toivo]

There are extensions that could do the job. Have a look at extensions in this category in the Joomla Extensions Directory (JED):
http://extensions.joomla.org/category/c ... iple-sites

[syser]

There are extensions that could do the job. Have a look at extensions in this category in the Joomla Extensions Directory (JED):
http://extensions.joomla.org/category/c ... iple-sites

Thanks a lot , sir !