Most agencies charge approx a months deposit for advertising ( plus vat)
Some agencies offer a management service too, this can be usually 5/6% charge of the monthly rent.
You need to decide if you are happy to deal with tenant issues yourself or do it via the agency and pay the agent a monthly fee including the initial advertising cost.
The agent will draw up contracts for you and the tenant to sign, make sure you are happy with the detail in the contract( pets, smoking, fine for non payment of rent, )
There is no guarantee that the agent will find a tenant for you and most contracts( from my experience) won't cover you for this, however if you get tenants in place and they decide to move out before your lease is up, the agent is normally liable to get new tenants in for you, for the remaining period.
Remember to register with the PTRB, this was never outlined to me as a landlord by the estate agent and it was months before I found out that you are LEGALLY OBLIGED to register.
Good luck