How should i pay income tax for variable income?

Hi,

I have last year i changed couple of companies and now i am getting variable income as a freelancer.

First Company: 34500 / month salary for 4 months

Second Company: Salary variable sometimes 31000/month, sometimes 10000/month for 8 months

Now i am working as a freelancer and getting less income from freelancer.com sometimes 7000, sometimes 1000/month.

Should i pay income tax? If yes, then how?

Please some body help me.