Welcome, Alex!
As Sue has said, I kept goldfish myself for many years.
Your tank is indeed a good size for goldfish:
• it would give 2 single-tailed goldfish like the one you have a lovely home (possibly 3 but the “thinking” tends to change over time to larger requirements per goldfish and so you might find yourself five years down the line and this being considered only large enough for 2) if they are to thrive and reach their life expectancy of several decades. Alternatively, you may know someone with a pond or do an advanced search on this site
www.aquarist-classifieds.co.uk to find someone in your area with a pond and prefer to move the goldfish there;
• if you prefer to keep fancy goldfish instead, then your tank would make a lovely home for about 3 of these.
While the fish may seem small now in proportion to the tank, they will grow – fast – so don’t be tempted to buy more than this! This article
http://injaf.org/care-and-information/the-goldfish-section/what-size-tank-for-goldfish/ explains all about it.
As for care and feeding of goldfish:
• goldfish tend to prefer slightly alkaline water but tolerate a wide range of PH and water hardness - you’d be best to try and get your water down to around 8 for PH (7.2-7.6 is usually ideal and you might get away with up to 8.5) and to soften it a little as well;
• frequent water changes (at least once per week, 30%, without fail) are vital, and more frequently if there is any trace of ammonia which goldfish tend to produce a lot of;
• very strong filtration is needed given how much waste they produce – either a couple of very strong internal filters or ideally an external filter, as they need to filter about 10x the volume of the tank per hour;
• a diet of flakes or pellets plus some veg (eg peas with shell off and chopped into small morsels, chopped courgette, spirulina, spinach) at least twice per week and live/frozen foods maybe once per week is helpful, as they (and especially the fancy varieties) can be very prone to constipation.
http://injaf.org/care-and-information/the-goldfish-section/goldfish-and-their-diet/ gives some advice on care and feeding of goldfish;
• try and keep the temperature of the tank down in summer – they do best around 20’C so you may need to do daily water changes in summer to keep the tank temperature low enough, otherwise they become lethargic and float near the top.
Hope this helps as a “starter for ten” at least but feel free to ask further questions if you have any.