To be honest, you could easily have used 50MB just downloading apps and syncing your contacts - I wouldn't be too concerned about that. Do you not have a Wi-Fi connection available?
In answer to your question, though, I would probably leave the main things on such as Google Sync and Email Sync, as these use quite little data but their use outweighs what little data they use. Then it's just a case of going in to all the other apps, such as Facebook, one-by-one and adjust their Sync settings - either disable entirely, or set the Sync for a long period of time (eg. 6 hours). This will reduce the data usage.
If you want to go the whole hog and disable all data transfer, your best bet is to go Settings > Wireless & Networks > Mobile Data > (uncheck Data Enabled). But again, I would point out that the first day's usage is unlikely to be reflective, and to give it a few more days to see what your daily usage is actually like.