So what you must have heard before is that Sheik's strength is not with her attack approaches.
Rather approach with movements or aerials and tilts at a distance (pre empty), baiting the opponent to approach and commit.
If you really wanna run in there and damage the opponent, a simple, effective and relatively safe attack (at beginner level!) is her Dsmash. It's really not that viable to walk up and Dsmash. But you can (if you didn't know) dash and at any time during your dash, crouch and then be free to input any smash you'd want. The crouch doesn't actually have to be visible, but simply inputting down and then emediatly inputting a c-stick action works.
Dsmash is the best option since it's hit box comes out the fastest. And it last long (three active hit box windows. Last almost a whole second!) and to top it off it has invincibility on the legs. Run up Dsmash can catch people off guard.
For a beginner some simple tips would be to try and shield you opponent's attacks and grab them. Remember to jab once or twice while grabbing, throw and follow up on the throw.
You can also look for openings while avoiding them/camping (keeping a safe distance from their approaches aka. defensive spacing) and punish their moves with lots of lag such as aerials that aren't lag cancelled, grab attempts, rolls or shielding that aren't acted out of (they shield and simply let go of it. HAS LAG) it's very normal to put up the shield and then letting go because you think you are safe.
Sheik's dash attack has an absurd range so it's easy to use as a beginner punish. Same goes for her dash grab.
TL;DR Don't approach with attacks and act on punish openings.
This is standard beginner Sheik meta.