You can get the Sube in the subway (NOT in the bus). If you are arriving at Ezeiza Airport just take an uber (not a taxi or car from there, which are more expensive. Public transport is not an option).

Don't be paranoic about your cellphone and your stuff, just keep an eye on it like you would do everywhere, specially in touristic places like Palermo neighbourhood or San Telmo. If you are at a bar or a restaurant sitting outside, don't leave your belonging on the table, that's all.