To report what someone said, you usually move the verb back one step in time: am / is becomes was, like becomes liked, will becomes would, can becomes could. If it's still true now, English often keeps the present (He said he is tired), but stepping the verb back is always safe.
Move the verb back one step
"I am tired." → He said he was tired.
"I work here." → She said she worked there.
"I will help." → He said he would help.
Not "He said he will help me tomorrow." — say "He said he would help me tomorrow.". Step the verb back one: will becomes would. (With something still true right now, like he is tired, you may hear the present kept — but stepping back is always safe.)
Try it: Mei: "I am busy." Report what she said. (Leave out the word you don't need.) Answer: She said she was busy. — Reporting it later moves the verb back a step: am becomes was, so She said she was busy. "Is" keeps it in the present, but reported speech steps the verb back, so leave it out.
say (no person) / tell (+ person)
Use tell when you name the person you're speaking to: tell + someone. Use say with no person after it. So it's told me, but said (not "said me").
"I'm tired." → She told me she was tired.
"I'm tired." → She said she was tired.
"I'll be late." → He told us he would be late.
Not "She said me she was tired." — say "She told me she was tired.". With a person right after, use told, not "said": told me.
Try it: He ___ he would call back later. (said / said to / told) Answer: said — There's no person named right after, so use said: He said he would call back. "Told" needs a person after it (told me), and "said to" also needs a person after it (said to me), so it can't stand alone here.
Change who the words are about
When you report someone's words, you speak from your point of view, so the people-words change to match. The speaker's I becomes he or she; we becomes they; my becomes his or her. And you points back at whoever they were talking to — if they meant you, their you becomes me.
"I lost my keys." → He said he had lost his keys.
"We are ready." → They said they were ready.
"I will help you." → He said he would help me.
Not "Lars said I would help you." — say "Lars said he would help me.". You're reporting it now, so switch to your view: his I becomes he, and the you he meant (you) becomes me.
Try it: Lars: "I will help you." Report what Lars said, from your side. (Leave out the word you don't need.) Answer: He said he would help me. — Lars's "I" becomes he, his "you" becomes me (he meant you), and will steps back to would: He said he would help me. "Will" stays in the present, so it's left out.