'Tell' has specific meanings.... you use it when what is said communicates new information, or instructs someone to do something.
It's a little confusing, but I think both words can usually be translated directly between different languages. What's your first language?
There are quite a lot of rules involved.... here are some.
'Says' sounds like 'sez'.
The past tense is always 'said', which sounds like 'sed'.
The past tense is always 'told'.
When you use 'tell' or told, you usually need to specify who it is you're telling....
I said "wash the car."
"Wash the car", I said.
I told Peter to wash the car.
I told him to wash the car.
Hasn't he washed the bloody car yet? Tell him again.
If you need more help, try using some examples.