To say something is a good idea, use should + the base verb. It is the same for everyone (I, you, he), and there is no to after it.
should + base verb
You should study for the test.
She should see a doctor.
You should try Farville Restaurant.
Not "You should to study more." — say "You should study more.". No to after should: just should + the base verb.
Try it: She ___ a doctor about that cough. (should see / should to see / should sees) Answer: should see — After should, use the base verb with no to and no -s: should see. "Should to see" adds an extra to, and "should sees" adds an -s that should never has.
shouldn't + base verb
To say something is not a good idea, use shouldn't + the base verb. (shouldn't = should not.)
You shouldn't eat so much sugar.
He shouldn't drive so fast.
We shouldn't be late.
Not "You don't should eat so much sugar." — say "You shouldn't eat so much sugar.". The negative is shouldn't, never "don't should".
Try it: He ___ so fast on this road. (shouldn't drive / don't should drive / shouldn't drives) Answer: shouldn't drive — The negative is shouldn't, not "don't should". And the verb after it stays plain: shouldn't drive (not "drives").
Should I ...?
To ask for advice, start with Should + the person + the base verb.
Should I call him?
Should we go now?
What should I do?
Try it: ___ I call Amina now, or is it too late? (Should / Do / Am) Answer: Should — To ask for advice, start with Should: Should I call her? "Do I call" asks about a habit, and "Am I call" is not a real form.
Try it: You look tired. You ___ rest, and you ___ stay up so late. (should … shouldn't / shouldn't … should / should … should) Answer: should … shouldn't — Resting is the good idea → should rest. Staying up late is the bad idea → shouldn't stay. Should for the good idea, shouldn't for the one to avoid.