I am installing a 100 amp sub panel beside a 200 amp main in my basement due to refinishing my basement and my main is full. My question is, is there a certain spot in the main that the 100 amp breaker needs to be installed that will power the sub panel?

No. The sub-feed breaker can be placed anywhere.

You need to use proper wire size for your code most likley #4 or # 2 copper. You need to tie in just below your 200Amp Main breaker. This may require additional screws or other hardware for proper connection. By tieing in below the 200 amp main should something happen this breaker will cut off. Be very careful and cutoff the 200 amp breaker before connecting. I you are the least bit unsure you should have a certified electrician perform this work. Be very cautous when working with this system as electricution can happen if you do not perform correct or with proper safety concerns.

Wayne, there is no effective reason to place the sub-feed breaker right below the main, nor is there any code requiring it.

Can you please explain what this means:

By tieing in below the 200 amp main should something happen this breaker will cut off.

Wayne,

The feeder from the main service panel board must be a 4 wire feed and it is sized according to Table 310.16. not Table 31.15 (B)(6). At least according to the 2008 edition of the NEC.

In this case, Copper would need to be #1 AWG @ 60(C) or #3 AWG @ 75 (C).