No no no to goldfish. They need a huge tank and are messy buggers - you'll always be cleaning them. Get a tropical tank with a built in filter and heater - so much easier to maintain. Go to a proper fish shop and have a browse (not pets at home, who know nothing about fish). Thru should be able to advise you too.
Goldfish are lovely and easy to look after IF you give them the right conditions. Spending hundreds of pounds on a massive tank and keeping a fish that lives for 20 years might be a bit much for a beginner, but if you're OK with all that then there's no reason why you shouldn't.
In terms of time commitment, I spend about half an hour a week cleaning out my 60 litre tank and a few minutes every day feeding, checking fish, tidying up plants etc. Setting up takes ages but if you put in the effort at the beginning then it's relatively simple to keep a properly stocked and maintained tank ticking over.
I'd start by figuring out how much space/money you have for a tank. You can either splash the cash or look out for second hand bargains (or freebies on Freecycle) but I wouldn't bother with anything smaller than 45 litres. 60-70 litres is a great size for beginners. It is a total myth that smaller tanks are easy to maintain. Definitely steer clear of all those rip-off beginner tanks that are only about 20 litres. They are death traps for fish.
Once you've got the tank and a place for it to go then you can fill it with water and just run it for a couple of days to make sure the filter and heater work and there are no leaks. When you've checked that then you can set it all up however you like. Some fish like certain decor and some are very picky about water conditions so if you've got a certain species in mind then check what they need and test your tap water to see whether it's hard/soft, alkaline/acidic.
When it's all set up and ready, you can start doing a fishless cycle. It takes about 4-6 weeks but don't skip it or buy any of those products designed to speed it up (they are mostly shit). A proper fishless cycle will save you loads of tears in the long run.
You'll need a source of ammonia- household stuff is OK as long as there's no other additives, some people just use fishfood and I've even heard of people just weeing in the tank I'd stick with the household ammonia personally, it's easier to control the amount of ammonia going in.
You'll also need a water testing kit that will test for ammonia, nitrIte, nitrAte, pH and hardness. Liquid kits work out cheaper in the long run.
Hope that's enough info for you to decide what to do next, if you have any other questions then just ask I'm a massive fish geek and more than happy to waffle on.
I started out with 2 goldfish in a bowl, I now have a 350 litre tank with 54 tropical fish in! If I were you I'd try to get a 60ish litre tank and keep some guppies in it. They come in loads of different colours (and you might get babies!) Or I would get maybe a 30 litre tank and have a single male betta (maybe with a few shrimp?) Oh I could spend hours planning it all! Lol. But definitely do a fishless cycle, is so much nicer for the fishies.