On the question of flash, it's generally okay to use the on camera flash if you need fill light. Having said that, you may hear that the lighting is "flat." Ultimately, you'll probably want to upgrade and get another flash like the SB-700, SB-900, SB-910, etc., which will allow you to achieve "bounce flash" by shooting the flash off a wall or the ceiling. Lighting is much better when you do this.
What you will find on this forum is people will give you great advice on making your images really POP, nailing the lighting, composition, references to reading materials that are very helpful (2Wheel is dead on with his 2 recommendations). If you are new to photography and this is your first DSLR, you are going to need to jump in the deep end and totally immerse yourself in this stuff to get a really good understanding. Once you get up to speed, I think you'll find this an excellent forum to learn. Gotta have thick skin though because nobody (very few) will sugar coat it for you. You want C&C, you'll get direct C&C. Stick around, read a lot, learn, shoot more images of that beautiful boy and have fun.
Welcome.