Regardless of if you want a pet, a dog for some working venue or a show dog the selection of the breeder will be the most important step in your puppy buying purchase

I will NOT address titles of the parent dogs, other than to say that the parents should hold a title in the venue YOU wish to participate in (ie if you want to show in conformation, at least 1 parent should be pointed or hold some show rating or title, if you want to compete in obedience at least 1 parent should be obedience titled, if you want a working ranch dog then at least 1 parent should either be one or hold herding titles etc...)

FIRST
the breeder themselves should seem trustworthy to you. They should be willing to answer your questions and no question should be deemed stupid. They should be a person that you will want a friendship with for the life of the dog AND they should be commited to remaining in contact with buyers for the life of the dog.

SECOND
they should be MORE than willing to give you email and/or phone contacts of previous purchasers as references

THIRD
If you go to their home, it should be fairly clean (though may be cluttered) and the dog areas should not be covered with feces (1 or 2 piles fine but not days worth). Dog crates should be clean. Dogs should not smell of urine or feces. Dogs and pups should not be afraid of their owner nor trying to kill visitors

FOURTH (and even if "all" you want is a pet this MUST still be done)
ALL dogs (Rottweilers) used for breeding (talking dogs LIVING in the US or Canada) should AT MINIMUM have OFA certified hips and a CARDIOLOGIST (or specialist) OFA cardiac clearance (NOT a practitioner one- look at the number C=cardiologist, S=specialist, P=practitioner) . Elbows should be OFA evaluated as well and should not have UAP, FCP or OCD. and DJD rating should not be higher than a 1.

Imported dogs that are LIVING and being bred in the US and Canada should have been redone using OFA (hips/elbows) and this is a REQUIREMENT if the breeder is an ARC member.

FIFTH
all pups should be no less then 7 weeks old when purchased, preferably 8+ weeks old (and in some states 8 weeks is the legal minimum) and should come with a veterinarian issued health certificate (even if this is not a State requirement)

SIXTH
DO NOT ever buy from a pet store or broker of any kind. Even if their pups do not come from a traditional "puppy mill" they will not have been produced by an ethical breeder because ethical breeders do not sell to brokers or pet stores (and they also do NOT sell them at flea markets etc)

SEVENTH
IF the breeder does not ask you a list of questions and seem to screen buyers, walk away. Screening buyers is just 1 indication of caring about their pups for life.
ALL purchase conditions and requirements (including what happens to the dog if you can no longer keep it)should be laid out in a purchase contract which you must READ before signing and if it contains anything you do not agree with do not sign it.

EIGHTH
websites are for browsing and information purposes only and should NOT be a PRIMARY factor into your decision process

NINTH
be aware that well bred pups in the US average $900 - 1500 and if the selling price is significantly above that for an 8-12 week old pup, you should probably walk away under MOST circumstances

TENTH
NON refundable deposits are also a warning flag. While keeping a portion of a deposit if you bail on a pup you committed to is fine, keeping an entire deposit is not (although keeping the deposit until that pup is resold is normal)