I have to agree with those who advised NOT to buy some cheap Kmart or Target bike.
They are heavy and they just don't glide along effortlessly like a good bike does.
I bought a Giant all ally bike about 12 years ago, just on the basis of a test ride.
After my previous old iron pig of a thing, it almost glided along all by itself, and that is something which makes you want to use it, rather than take the car.
I have since bought an electric bike, but after the novelty wore off, I mostly still ride my Giant bike. It's just more satisfying.
Also, get something with front suspension but not rear suspension. Front suspension saves you from a lot of stacks when riding over unexpected obstacles, like curbs, small animals, potholes etc., without wasting too much energy.
Keep in mind that the energy for every compression of the suspension has to be supplied by the rider,.. you,.. so rear suspension is just wasted effort. Not so much for front suspension because they don't compress as often, only over bumps that you really need them for.