Why do you think this? You'd be surprised how many people turn up here saying that they think they've been hacked or their phone has a "virus" (sic) when actually it's nothing like that, so I do have to ask.
The most likely ways for anyone else to gain access to your phone are:
1) you let them physically have access to it (allowing them to install something)
2) you download and install apps from random sites on the web
3) your GMail account is hacked, and the phone accessed via that
It is really, really unlikely that your phone would be hacked via any other route.