I want to buy a low price bicycle and I've not used any till now. what is best to start at budget price? I want to use in street and simple level mountains. I'm 30 years old.
If properly assembled and maintained, the safety level of bikes tends to be fairly consistent. While technological advancements are consistently being made, the fundamental safety aspects haven't changed that much in the last 50 years. For example, brakes have gotten a better, but well maintained brakes from the 70s are perfectly reliable.
To ensure that your bike is properly assembled and maintained, you'll want to either get it from a reputable bike shop or take it to a reputable bike shop after purchasing it. If you get it from the shop, great. You're good to go. If you bought a bike from another source (such as a used bike or a big box store), most shops will do a safety inspection, perhaps charging a fee. You would, of course, need to pay for any additional maintenance.
With that in mind, you can shop around for what's cheap. If you go to a shop, it's easy. The staff will likely help you find something that fits your needs at a price you're willing to pay.
If you look for something outside of a bike shop, factor in the cost of a safety check from your bike shop and any repairs that might need to be made.
Given your goal of mostly riding on the roads with a bit of light mountain biking, you'll probably want a hard tail mountain bike. It'll serve both those purposes and still be pretty inexpensive.
A note on big box stores
Incidentally, I would recommend avoiding big box stores. While those bikes are cheap, the components are often very poor from the factory and then they're poorly assembled at the store. They're so bad that they're often derogatorily referred to as "Bicycle Shaped Objects" or BSOs for short. A good mechanic can turn a BSO into a decent bicycle, but if you're not a good mechanic yourself it'll cost you more than it's worth.
If you absolutely have to go with a bike from a big box store, take it into a bike shop for a safety check and expect to have the brakes adjusted at the very least. It's likely that you'll also want them to true the wheels and adjust the shifting. Those are the most common problems I see on big box bikes. If you inquire about the cost of those tasks ahead of time, you can fact that into your purchasing decision.
Onging bike use require regular maintenance to keep them safe. You could buy a bike in good working condition, but if you do not maintain it, it will become unsafe sooner or later. If you learn to maintain a bike yourself, it will be a lot cheaper than getting a shop to do it, and a lot safer than not doing it.
As far as purchasing, local bike co-ops, if you have one, often give away bikes or sell them at very low prices. Usually older bikes but always in good working condition. They will help you with bike repairs. Absent that, as already mentioned, a used bike from a local bike will be the easiest and safe option.
As around people you know, many have bikes that are unused. If you buy a bike privately, your local bike shop (LBS) will be able to do a safety check and service on it. This might be the cheapest option.
Your safety will depend much more heavily on wearing a helmet and biking in safe locations (bike lanes, no cars, etc.) than on the intrinsic safety of the bicycle. Buy a new helmet, and adjust it properly. Having said that, if you want even more peace of mind, I would go for a steel frame less than 5 years old, and avoid old aluminum frames due to potential metal fatigue issues.
If you buy a used bike: Check that the brakes are in good order, that the chain is not super old and rusty. If you ride at night or low light conditions, use reflectors on the wheels. Use blinking lights to indicate your presence; red lights in the back and white in the front, not excessively bright.
If you're learning to ride a bicycle, completely avoid roads with cars until you know what you're doing. Otherwise you might loose your balance or try to avoid another bike and accidentally swerve into traffic.
At the beginning, set your seat post height so you can have your feet on the ground while sitting. Eventually you can raise it a bit more to the proper ride height.
As for "cheap", that is very relative. If you search used forums like Craigslist or OfferUp in the US, you can probably get something decent for $50 to $100 or even free. But maybe your budget is $500, in which case you have a lot more options. If the seller is selling a $2000 bike for 200, it's probably stolen, so avoid deals that sound too good to be true.
Good luck and enjoy this wonderful invention.
Start by borrowing bikes from friends/family/coworker or neighbour, and see what you like.
There's probably a misconception that you need "all the bike" to ride because that's what is in the shops sold new. Any bike can be ridden on the street and will cope well-enough with "simple level mountain" which I presume is graded gravel paths and not a lot more. You don't even need suspension to ride things like that; whereas it's implied a full-suspension bike is needed.
There's nothing wrong with riding used bikes - they cost far less than new, although its up to you to work out if there's any damage/wear before buying.
As for "safest" that is partially the condition of the bike, but is also a lot to do with the mindset and abilities of the rider.
If you lack spatial awareness while riding, or fail to spot what will turn into an obstacle to avoid, you're going to eventually have a bad time.
Example - presuming that a green traffic light means its safe to proceed. Any competent road user will flick an eye in either direction and react accordingly.
Don't let that put you off - the risk is the same if you're riding a new or old bike. And that comes with practice.
What you can do is learn the basic M check and periodically test your bike, once a month is considered good.