Hey simon10!
The first thing you want to do is research.
What i do is research what specific fish i want to keep and the rest falls into play. When you know which fish you want, then you know which equipment will be right for them.
Things to look for are: Habitat, diet, behaivor, adult size.
When buying a filtration system, i recommend getting one that is graded for a bigger tank. For example, say you have a 10 gallon tank, i would get a filter that is used for 15-20 gallon tanks. I have a 30 gallon tank with 2 filters that are graded for 40-70 gallons (i think thats 300 gph all together on a 30 gallon tank!) and the water quality stays great. I have had it set up for 4 years now and never have a problem with water quality.
Lighting greatly depends on whether you are going to house live plants with your fish. Live plants themselves also need the same amount of research as your fish do. It may be a bit more costly also to keep live plants as they require their own nutrients and requirments themselves. I believe the best tanks with live plants are the ones that were planned to be live planted tanks.
For gravel, it may also depend on the fish you keep and if live plants are going in. Some fish like to "dig" or burrow into the growund while they scavange so giving them a soft, fine substrate will make it easy for them and will not damage them. As for plants, it is higly recommended to get a substrate that is full of the nutrients that they need along with light and C02.
There is alot of information out there and being apart of a fish keeping forum is a great ide as alot of the information you read about on the internet is based on personal experience of each aquarists. Being apart of this forum will allow you to pick the brains of experienced aquraists that know what you have been through and will help guide you through your new endeavors.
If anyone want s to ellaborate or see any fault in any of my information, feel free to correct my mistakes.
Good luck Simon10!
Best regards,
live_the_wild (Matt)