What size do you need? Many experienced carpenters use a 16 oz. claw hammer for most jobs, but also carry a 20 or 22 oz. framing hammer for their heavier nailing jobs requiring big spikes. For lighter work, a 13 oz. curved claw hammer can be useful. Even a 7 oz. hammer will come in handy for smaller jobs like making picture frames.
To keep tools rust-free, keep them clean and dry by wiping them off with a dry clean cloth before you put them away. To combat rust and corrosion that is caused by humidity, occasionally coat all metal surfaces with light oil. If you live in a humid climate or keep your tools in the basement, a dehumidifier will help.