How can you join second life without mobile or credit card?

The most minimum requirement for the ability to sell and purchase Linden Dollars is some form of real life payment. Options aside from a credit card would be something like a Debit Card, or a PayPal account.

There currently is no way to purchase or sell Linden Dollars without having some form of real life payment associated with your account.

It is possible to earn Lindens within second life and use them to buy things without any real life payment details. To do this you will need to provide a service or create and sell goods to other players.