This depends entirely on your budget and what you want out of a bike. If I had to make one choice based on having absolutely no information on what you actually want, then I'd say budget $600-1200 on a new bike that you buy from a local bike shop (LBS). Don't buy used if you can avoid it, mainly because basically every used bike I've ever bought had some sort of hidden gremlin somewhere. Besides, if you buy from the LBS you'll usually get nice perks (be fitted properly, usually discounts on maintenance, a place to take it to if something isn't right, and more).

Different frame materials have different advantages/disadvantages.

Carbon is very stiff (a good thing), but also great a dampening things like road vibration. Modern carbon is incredibly durable, but it's also quite pricy for the good stuff (I don't like cheap carbon), and it's not very easy to repair (though in some cases it can be done).

Steel has incredible longevity. It tends to ride a bit smoother than Aluminum, but it can flex a little bit, meaning a little bit of energy is wasted flexing the frame rather than propelling you forward. It tends to be a bit heavier than the other options, though that's not always the case. One great thing about steel is that dents and such aren't a big deal and it's repairable. Steel is also low cost.

Titanium is like a nice compromise between carbon and steel in a way. It's stiffer and lighter than steel and good at vibration dampening, but it's also possible to weld if needed (though not as easily as steel is as it requires special equipment, but it can be done). It's also very corrosion resistant. It's VERY expensive.

Aluminum is cheap, quite stiff, and light, but it tends to vibrate a lot (which is why aluminum bikes often come with carbon forks). One of the bigger downsides is that it's not very repairable and it weakens structurally as you ride it, so you can basically guarantee it will fail after enough stress cycles, however, it may take quite a long time to get to this point. It's not uncommon to get a decade or more out of an aluminum frame, there are way too many variables to say when this failure will happen.