Is this your first house, or have you simply moved house? If you've moved house then do you not have any meter readings from your old place to work out roughly how many gas/elec units you use?
If it's your first house, then you're just going to have to guess your amount of usage (I imagine there are calculators online to help with this). Obvious questions:
How many of you will be living there?
How much time will you be at home?
How well insulated is the house?
How big is the house?
Is it detached?
Does it have gas central heating or electric?
For reference I live with the missus in a 2 bed semi. It's got decent loft insulation but no cavity wall insulation and fairly rubbish double glazing. We pay an average of £56 a month for both gas and electric with EDF (Blue Price Promise fixed until May 2014).
If you've got no idea how much you're going to be paying, I'd suggest you sign up with a provider that doesn't tie you into a long contract (just incase it turns out to be a naff tariff). After a few months you'll have a good idea of your usage then you can switch to a deal that better suits you. Switching is pretty straightforward so I wouldn't worry about it.
As for which energy provider to go with, I would just go for the cheapest as I think they're all much of a muchness. I've been with EDF and British gas who have both been ok, but I bet the next poster will come along and slate them!
Finally, make sure you look at topcashback/quidco before signing up as you can often get a good chunk of cashback.
