Well, you are asking the right questions.
Step 1 - cycle the tank. You need ordinary bottled ammonia - available at your local hardware store. Make sure that it includes NO surfactants nor perfumes. Just ammonium hydroxide and water. Or you could get some ammonium chloride. There is a thread in the Beginners Resource Center (linked in my sig) on where to find ammonia in the UK.
You will need a test kit - API makes a nice kit with pH, high pH, ammonia, nitrite and nitrate. Very affordable to buy that kit rather than the individual tests. Be sure to check out the article on fishless cycling (also in the Beginner Resource Center).
You will need a heater and a filter. Opinions vary on both. What size tank are you considering?
The good news is that the cycling process will take a few weeks which will give you time to research the rest of this.
Planted tanks come in a variety of styles... and you will want to work out what type of planted tank you want to have before you choose your substrate and lighting. There are high tech which include CO2 dosing, daily fertilizing doses and high lighting. Then there are low-tech that require very little effort from the fishkeeper other than standard fishkeeping maintenance. Remember that your plant choice options will be limited based on what you are looking at in terms of lighting, CO2, and ferts.
You can start the cycling process on an empty tank - other than a heater and filter - while you work out the other details. You will have about 3-6 weeks to research everything.
Tell us about your water - pH, gH, kH, etc. as well as your tank dimensions, and your personal taste, and we can help to steer you in the right direction.