Make sure that you thoroughly read the small print as some clauses can unfairly favour the landlord or agent. If there is something that looks a bit dodgy or which is in legal terms that you don't quite understand, get the advice of a seasoned pro, a solicitor or the Citizens advice bureau. Normally the landlord or the letting agent will provide the tenancy agreement. You must read the document thoroughly. The tenancy agreement is normally drawn up by a solicitor or according to a set model that has previously been devised by a solicitor on the lettings agent's behalf.
Some people advise against landlords using off the shelf tenancy agreements.
The only possible exception to this is where two friends have a landlord and tenant relationship, but even then a written agreement is strongly advisable to protect against unforeseeable changes in circumstances. In theory, tenancy agreements can be verbal. These should be as legally binding as written ones. In practise, relying on a verbal agreement to stand up in court is a dangerous practice. When it's your word against the landlord's it's not always the person telling the truth that wins.