There is a lot of info available! Sadly some of it is total bollocks so you have to do tons of research at the beginning. It's worth it though, because if you start out right it'll save you loads of work and money in the long run.
120 litres is a great size, it's not so big that it'll take hours to clean but it'll give you lots of choice of fish.
If you just want a tank of small, mixed tropical fish then you might want to go for a plug and play type tank with the filter and heater already installed. Juwel Rios are popular and have a good reputation, I've never had one though. Don't get any of those sphere shaped or novelty type tanks, they are all crap.
Once your tank is all set up, you'll need to run it without fish in for a few weeks. This explains all about how to make the tank ready for fish. There are all kinds of products available that claim to speed up the process but not all of them work so best to just do it the old-fashioned way and wait. This will also give you time to plan your stock.
Different species of fish like different conditions. If you don't have anything specific in mind then the easiest thing to do is probably to choose fish that will suit your tap water. You'll need to know the hardness and pH and then that will narrow down your choices. A water test kit like this is a good bit of kit to have. You need to be able to test for ammonia, nitrIte, nitrAte and pH at the very least (there's other stuff you can test for but those are the basics).
If you do have a particular species in mind then you may need to set up the tank with that species in mind.
My main advice would be don't rush anything at the beginning! It takes a long time to set up a fish tank and stock it, but once it's done then it's pretty low maintenance. If you cut corners then you end up with diseased fish and you'll have to spend loads of time (and cash) trying to fix things.
If you just want to have a browse of different species then this is a good website with accurate info (some websites are very dodgy with info!) and PFK magazine is good for inspiration and articles.
Hope that helps a bit :)