Looking for tips

So I am about to get my licenses and just got a 2007 Nissan frontier se 4x4 90,xxx miles. And well I've got a few questions. For better sound quality would you recommend changing speakers or a new head unit and speakers? Also with no change of tires or maybe a little more aggressive what is a good mild lift? And at 90,xxx miles any suggested maintenance (I've done oil and brakes). So far I've cleaned frame and under coated and I've got a tool box. Also keep in mind I am 16 so budget isn't huge.