You can click on contact's or opportunitys tab then click on new. On the layout of both objects you have lookup field to account called Account name. From there you can choose to which account you want to link the opportunity or the contact.
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You can do as Ahmad J. Koubeissy suggested , in addition to that you can also write a trigger that will create contacts and oppprtunity as soon as account is created
You can click on contact's or opportunitys tab then click on new. On the layout of both objects you have lookup field to account called Account name. From there you can choose to which account you want to link the opportunity or the contact.
Kindly mark as best answer if this helps you
Best regards
You can do this via Trigger. Below is a code for the same. I have tried in my Dev org its working fine.
Apex Class
Apex Trigger : Regards,
Akshay