As I understand the Illinois rules, service by mail can only be done if the Defendent lives in the state. If the Defendant lives outside the state, you need to get the appropriate copies of the summons from the clerk and arrange for them to be served on the Defendant (in California).
When served, the summons will be returned back to the court with time and date of service. Your case will then be set for trial.
Now, for your other questions:
Q1) "is the person obligated to come to court if they accept the summons served by certified mail."
A1) As I understand it, certified mail is not acceptable for out of state service (see above). As for the 'obligation' issue, no, the Defendant is not OBLIGATED to show up, but if he/she doesn't show then you will get a default judgment.
Q2) "Also what will happen if they don't show up for court would they be in default."
A2) They won't be in default, but you would receive a default judgment.
Finally, one other thing to consider when you are thinking about suing an out of state Defendant. If you win, how will you enforce the judgment (the courts can't force payment). Unless the Defendant (now the judgment debtor) has assets in your state, you have nothing to sieze and will have to transfer the judgment to California to try to enforce it.
Kind of makes the whole question of "how to serve" moot, doesn't it???
