maternity clothes?

Nikki
This is my first pregnancy and I am wondering when will I need to start wearing maternity clothes? My husband has commented that I could start shopping for maternity clothes with our clothes budget if I wanted. At 16 weeks, I don't really have any idea what I should invest in and what I should just skip on. Some of my jeans are snug but nothing that I can't fit into. All of my tops still fit but my boobs have kinda exploded so i'm selective on what I wear. 
I would appreciate anyone's suggestions on maternity clothes- what I should buy? What sizes do I buy? And any other tips maybe. Thanks!