Lechatchila, you should be very careful to be particular about enunciating each letter of each word of Shema. It cools off Gehinnom. But if you were not, you are still yotzay. (ShA 1 – MB 2)
The best way to do the mitzva of Shema is to recite it in Lashon Hakodesh. This applies also to Tefilla, Hallel, Birkas Hamozon, Kiddush and berachos for mitzvos and food. If you say it in any other language you can fulfill your obligation provided that the translation is accurate (some say this is not possible these days), you understand it, and it is a spoken language in the country you are in. But when recited in Lashon Hakodesh you are yotzay even if you don’t understand more than the first possuk. (ShA 2 – MB 3 – BeH “yachol”)
Lechatchila, you should hear the words of the mitzva that you are reciting. Bedieved, you can be yotzay with silent recitation. Merely thinking the words does not count. (ShA 3 – MB 5,6)
If for some reason you cannot speak (e.g. sick, recovering from vocal operation, touched a filthy place) you should think the words, and Hashem will fix your reward for that. Technically you did not fulfil your obligation, therefore the moment that your situation changes you are obligated to recite the words normally as long as the mitzva hasn’t yet passed by. (ShA4 – MB7 – BeH “v’af”)
If you are in a filthy place, don’t even think the words of Shema – for it is forbidden to ponder words of Torah in such places. Rather feel upset that you have no way to fulfil your obligation and Hashem will fix your reward for that. (Rama 4 – MB 9)
When the chazzan reaches the Shema, he must sound out his voice so the entire congregation hears the signal to coronate Hashem in unison. (ShA5)
If you are not yet daavening and you hear the congregation reciting the Shema – join them. You should have in mind not to fulfill your obligation now, so that your Birchos Krias Shema will be accompanied by the mitzva of Shema d’oraisa. (BeH “k’day”)