Does your crotch hit the top tube with your feet flat on the ground? That will hurt.
A woman wants an inch between her crotch and the top tube when standing over the bike. A man wants an inch and a half from his testicles. Both of you want a bit more room on a mountain bike.
Set the saddle level.
Put the saddle no higher than you feel comfortable. You can worry about speed and efficiency later. Right now we don’t want you falling over when you come to a stop.
To get the saddle height close to right, go for a ride with the sun at your back. Ride for a few minutes to warm up. Then on a quiet straight pedal with your heals. If your shadow’s hips waggle, the saddle is too high. No waggle: try higher.
If you have big feet, you can set the saddle even a bit higher.
If you have platform pedals, you can pedal on your insteps to get a feel for a higher saddle. (Also useful when you wear thicker shoes.)
Some people have their saddles too high. You can see their hips waggle as you ride behind them. They have the mistaken idea that their legs should be straight at the bottom of the stroke. It will hurt their knees, slow their stroke, and grind their crotch.
On the road a mountain bike should have the same saddle adjustment as a road bike. On the trail you want the saddle a bit lower.
Does the saddle fit you? Most women have wider hips than men. A man’s saddle won’t feel good for long. Men have narrower hips: a woman’s saddle will chafe.
Where do you bend? At the hips or at the waist? Stand up, keep your legs straight, and reach for your toes gently, don’t pull yourself down. Can’t touch your toes, then you’ll rotate at the hips and grind your crotch. Look for a saddle with a groove in the middle to ease the pressure. Many women find a grooved saddle more comfortable.
Some small bikes have standard length cranks. If you ride a lot, you might want to buy shorter cranks.