Here's the advice I usually give people when they ask me. And being the kind of guy I am I normally start by asking questions.
"What kind of riding do you want to do"?
Let's assume the answer indicates a road bike. In other words, is a road bike of some flavor right for you? Think about it.
"How much do you want to spend"?
If you don't have gear, set aside at least $100 and better $200 for a helmet, shorts, and other accessories. For the bike itself, I say $500-600 US will get you a new road bike at a shop that will let you ride hard for 2 years. And remember, $730 dollars is a dollar a day for 2 years. Assuming you keep the chain clean and lubed and replace it once, you can put 10,000 miles on that bike, though that would trash it out unless you did more maintenance. For reference, if you ride 1000 miles your first year you will find yourself in a lot better shape.
Personally, I spent a grand on my latest bike, even though I could have spent more if I wanted to. I would have liked to have spent 3 grand sure, but really, it's just a bike. That other 2k will give me a lot of better returns elsewhere.
After price, I tell people to go to shops and test ride. Test the bike and interview (casually) the shop. A good shop is
needed for newbies (unless you have a lot of years MTBing, in which case you are probably a good jackleg mechanic).
When riding, your position should have your torso at somewhere close to 45 degrees to the ground when on the hoods (top of brake levers). You will ride there a lot. And yes, you can use the brakes with your hands there.
Adjust the saddle. With the bike held upright by someone else, and you holding the bars and sitting in riding position, put one pedal down at "6 O'clock". Your heel should scrape it lightly when you swing your leg. When you put the ball of your foot on the pedal, your knee will bend to what will eventually be close to the perfect extension. It is not that yet. Lower the saddle a half inch (or a centimeter), and measure when you do to make sure you lower all the saddles on all the test rides the same amount.
If you really want to be sure, make sure the saddle is in the middle of the rails on each bike too.
If you do all that, you will not only be in close to a perfect riding position, but better able to judge the BIKE, and not the effect of your position on the bike. Pick the bike that feels best from the shop that you felt the most comfortable with in terms of service.
It's that simple
That is what I tell people when they ask me.