Well they have to be oppisite genders and you have to connect them a lot. They have to be adults and buying the love potion (honey) could help a lot too! You'll have to connect them a lot for about 2-3 days though. Breeding take time and takes a lot of patient. Hope this helps!
Breeding can stil lhappen even if you don't connect. When your Tamagotchi is 6 or 7, the Matchmaker comes. She will ask you if you want to love or not love a Tama of the opposite gender. If you choose yes, your Tama gets married and have a TamaBaby. If you choose no, your Tamagotchi won't get married, turn to an old folk when it evolves, and loose 1 happiness heart.