Yes, it's a good idea to put it in the lease. Remember that the deposit must be put in an interest-bearing account for the duration of the lease and given back to your tenant, plus the interest it has earned when the tenant moves out.
If, however, your tenant still owes you money on moving out, or if the property has been damaged beyond normal wear and tear, you can use the deposit to pay for repairs or to cover the money owed to you.
It is common for the landlord to require one month’s rent or a double deposit (2 months’ rent). The landlord could even require a triple deposit if the tenant's credit report is considered risky.
However, once the amount of the deposit has been agreed upon, the landlord cannot demand a bigger deposit during the term of the lease – unless the tenant agrees, or the lease agreement makes provision for a top-up when the rent increases
Practically, the landlord should not hand over the keys to the property until the agreed deposit and first month’s rent have been paid (and cleared in the case of a cheque).