Not much to add but when you buy the heater, make sure it is for vertical use and not too long. Your tank will have a calorifier inside so you need to shine a torch inside the tank to check how long a heater you can use. Has to be short enough not to foul the calorifier.
If there is an electical 'spur' it should be 13 A which is enough for a standard 2.5 - 3.0 kW heater. Use proper heat resistant flex of at least 15 A capacity and if there isn't a swirch, put one in and make sure that is also 15A.
Finally, if you aren't absolutely certain you can cope with all that, don't even go there, buy yourself a qualified electician. Don't become an inadvertent 'Live wire!!!

Docusk