i have Iphone 3Gs which is factory unlocked so i am using all the carriers in it (i.e i can use all sim cards like airtel, vodafone) and the IOS is 6.1.3 and modem firmware is 05.16.08, now i want to jailbrake it to use cydia and its features.

I want to know that
1) if i jailbrak it will effect my phone?
2) how to jailbreak it?
3)how evere there is no IOS updated for 7 but if in futer if then how to do it?