Do you need to use an agency? I do it myself, and thats fine (but I am self employed so I am flexible).
You need to have the electrics up to grade and the gas tested and get a gas safety certificate every year.
You need a deposit, it varies by house and area what people take - I take the equivilent of 1 months rent (some take 2). This has to be put into a government backed deposit scheme. I use the deposit protection service. Then on moving in day you want a months rent in advance.
You need LL insurance - and you need to shop arround, what they cover varies alot. You need a policy that covers malicious damage by a tenant - mine didn't and I have practically had to rebuild my house using all my savings and then some. Price varies on property and area etc so your best off doing a few online quotes for an idea on that.
If you use an agency they will check refrences and do a credit check on the tenants. I don't use them so I vet them myself - do it well. Don't ring mobile numbers, look up a landline etc.
Will you accept DSS? Thats something to consider, there are pros and cons to doing so.
What type of property is it and who are you looking to let to?
Is it in good condition? Between tenants I re carpet and re paint. I paint all walls and ceilings white and magnolia, and get beige 'landlord' carpets. (Seriously go into a carpet shop and say its a rental and they show you the rental house carpets). They are cheap, I'm just doing mine now and whole house of carpets and lino including fitting is £645.
You need a pot of money. If you are going to be a LL be a good one. You are paid for a service, so provide a good one. Repair/replace as soon as you are made aware of anything, hence the pot of money (a few thousand at least).
Once the tenants are in IMO I leave them alone, untill they get in touch with me. There are mixed views on inspections - some like them to see if there are any repairs/problems etc, others like me would rather let them be and trust that I make myself available to them if they want to report anything wrong.
Don't get too involved, I care about what the house is like when I let it and what its like when I get it back - what happens inbetween is their business.
Also be aware that BTL mortgages are more expensive than a normal one, and that intrest rates etc only have one way to go in the future. You can put the rent up but only so much, so make sure you work out your maths well.
I make sure that from my income I can afford to pay the mortgage and insurance on my rental house if it is unlet.
And finally don't forget Mr Tax Man 