Sounds like you are on the right track! You are right to ignore the shop's advice to bung some mollies in after a couple of days- that's quite old fashioned advice, there really is no need to use fish to cycle a tank.
This article explains all about the nitrogen cycle. It's fairly simple, just bloody long and boring You'll need to add a source of ammonia to start the cycle, some people use fish food or you can use household ammonia. The filter needs to stay on at all times, as soon as you switch it off the good bacteria will start to die off so keep switching off to a minimum (just when you are cleaning it and for a short a time as possible) and never, ever use tap water to clean it because the chlorine in tap water will kill the good bacteria.
Dechlorinator is an essential, not sure what nitrAte stuff you've got though- what does the label say?
You'll know the cycle is finished when the ammonia and nitrIte have risen and then fallen to zero, usually between 4-6 weeks. You'll need a test kit to monitor the water (don't rely on anyone else to do it for you). Liquid test kits cost more to buy but work out cheaper in the long run. The API or Hagen ones are popular. You'll need to be able to test for ammonia, nitrIte, nitrAte and pH.
I'd switch the heater on now as well to get everything settled before you add fish. You can add plants now ,it will give them a chance to get established before you put the fish in.
Fish choice is a bit dependent on pH and hardness. If you have very hard alkaline water or very soft acidic water then that will limit your choices slightly. I would suggest 70litres isn't big enough for mollies anyway. You could have a shoal of one species of small fish in there and a few shrimp as well if you fancy.
Very excited - got a second hand 70l tank off ebay yesterday and plan to set it up for tropical.
We also went to a big aquatic store and bought some dechlorinator and something to do with nitrates (?) can't remember the name but have written down instructions.
I understand we need to run a full cycle before adding fish, but not sure what a "cycle" is. How do we know when it's done?
Also do I leave the filter running for the whole 6 weeks, or do I only turn it on once fish are in there pooing?
I assume I only need to turn the heater on a few days before, to get the water up to temp. Also assume we can put plants in straight away?
Fish shop staff advised just putting mollies in to start with - in fact they advised doing that within 2 days but I shall be ignoring that! Are mollies the best starter fish? Eventually would like tetras and a few others but plan to keep it fairly simple for first few months.
I'm sure I will have many more questions but can't think of any more right now.