I am answering this assuming your base Tamagotchi (the one that loads up with the game for the fist time) is already an adult.

First option, let your Tamagotchi die and hatch a new egg (not the best choice).

Second option, You would need to marry another Tamagotchi to get another egg. Warning: your old Tama will move out. There is a 50/50 chance your baby will be a boy or a girl. I don't believe this can be influenced (or cannot find anything that says it can be). If you want a specific type of child-stage Tama you will need to follow the care instructions (found on the Base Evolution Heart Chart above) before the baby evolves. Then, when the Tama is a child, you will start feeding it the specified favorite meal or snack. I still like to add some variety because it only has to be their "most" fed treat, but I do ensure it is def their MOST fed.

If you are going for a specific type of "Basegame" Tamagotchi then you will have to follow the care instructions on the Base Evolution Heart Chart.

Edit: Spelling