There are 3 options for you.
You can sign up at a poker room like 888 and use their no deposit welcome bonus to start with and play a few small buy in tournaments.
You can sign up at any poker room and use their free option, this is like the real option however you are given a free balance and can use that to enter free tournaments.
You can sign up to somewhere like Americans cardroom. They offer
freerolls on demand, this means that there are always freerolls for you to register into and play for free and have the chance to win a little bit of money if you finish in the top 10.