How to file last year ITR

No you can not file now, every year you have to file before due date. 

No... We can't file ITR for AY 18-19...

If your income is below 250000 then you need not required to bother about that

if your income is above 250000 and have tax liability and if such tax liability adjusted against your TDS or Advance tax then also you need not worry

if you have tax liability and you not file your ITR Don't worry IT Dept itself will send notice to you to pay tax liability with it interest and penalty