Most landlords will require tenants to pay a deposit equivalent to one month or six week's rent as soon as the tenancy begins (the day you move in). They will also usually require the deposit as cleared funds in their bank account before the day you move. You should therefore look to having at least two months rent when looking for accommodation.
Landlords now have to place your deposit in one of three Government-approved Tenancy Deposit Protection Schemes
If you are on a low income, starting a private tenancy may be difficult. Ask family if they may be able to assist you with this initial payment to secure accommodation.