I'm a LL and have self let and managed for 10 years, and just now am looking into using an agent. In fact I have met with 4 today!
You will need an EPC (energy performance cert). Get them online yourself. Cheapest I found was £50. Agent will charge you in excess of £100 for this.
You need a gas safety cert every 12 months - check out local plumbers for this.
Get landlord insurance. Pay for a decent cover. Make sure it covers malicious damage. I have learned that the hard way. Last tenant destroyed my house (literally house was stripped back to the joists) and I didn't have malicious damage cover so I had to pay myself. It has nearly finished me financially.
Get a few agents in. I think I have decided on a local company but I was quite impressed with Your Move who are national. They did a good rent guarantee scheme, its 2.5% of the rent each month and basically if they don't pay after the first month they give you the rent for 6 months and £15k legal fees to get an eviction.
From my personal experience - let unfurnished.
Take a very detailed inventory, with photographs (agents can also do this, Your Move charge £120). Get tenant to inspect with you and sign it.
You are going to have to detach from this house. Even good tenants will not treat it like your home, they treat it like theirs (which is only right, but can be hard to cope with).
Personally between each tenancy I allow enough money to repaint and re carpet. Usually I only re paint and have the carpets cleaned, but I allow money for it.
Remember you have to declare this as an income for Mr tax man. Also look into what costs against the property are tax deductable, like mortgage intrest and agents fees. Not all repairs are.
Have a look into local tradesman/companies like british gas who do maintenence contracts for houses. What I am going to do is let the house fully managed, but without repairs. I will be informed of repairs and get my trades to do them. It will be cheaper. The only exception to this is that they have a £100 max to use in an emergency if they can't contact me to have an emergency trade come out to make the property safe.
Be wary of their costs. It seems standard that that will authorise works up to £100 without your permission then bill you. Be aware that may mean a £10 part and £89.99 labour to their handymen. This is why I have opted out of this.
There is probably loads more so ask any questions, there are a few LLs kicking around here and we can usually point you in the right direction 