Can you tell me how session beans call entity beans to make transactions?
Do you mean "transactions" as in "I want to rollback if something goes wrong" or as in "I am doing some operation that the regular folks call a transaction"? I am assuming the latter...
1) Get an InitialContext. If the beans are in the same application, a simple new InitialContext() should do (otherwise you'll need to give some configuration parameters to the constructor).
2) Make a JNDI lookup for the entity bean's home interface. This could be done either by using the full JNDI name (e.g. "ejb/com/mycompany/ejbs/MyEntityBean"), or a logical JNDI name (e.g. "java:comp/env/ejb/MyEntityBean"). This depends on your ejb-jar.xml.
3) Use the home interface for creating/finding the entity you want
4) Invoke methods on the entity bean.