Probably the most often question I get asked is... "I want to build an Rov but I don't know where to even begin, can you point me in the right direction?"
I know where your coming from... I was in the same position when I started, except back then (yeah I'm dating myself here) there weren't any forums or even websites about home built Rovs and very few sites about commercial Rov's even. Today there are a ever growing number of personal web sites and forums dedicated to this hobby and seeing what others have done (and/or fail at) is a great way to learn. So to start a would suggest looking over some of the links I have on my site (Seen Here
Another great way to start is to pick up a copy of "Build Your Own Underwater Robot and Other Wet Projects"
by Harry Bohm and Vickie Jensen. Commonly referred to as the "Yellow Book". You can purchase this book from westcoastwords
for about $20.00. This book is written more towards kids but it has a lot of great info in it for the beginner (no matter what your age) and is well worth the money. There are plans for two different Rov's into it too, the first being the Seaperch, (a very basic Rov) and is meant as a way to get you feet wet. (so to speak) The second Rov, The Seafox which is a little more advanced and a great way to start. I've built a Seafox and then after many successful dives I upgraded it to be even more advanced and to this day it is still my main dive platform.
Another option is to start with a kit, of course the only kit I know of is the "Rov in a Box" from !nventivity
I haven't seen one of these personally but basically everything is supplied and you just have to put everything together to have a basic starter Rov.
There is the Mini ROV kit by MadLab
sold by Apogee Kits but this is more of just a toy, but still worth a mention.