Do your research on google. See the below site for a start. Then you will have specific questions you can get answered here.

I have a little 50 gallon (big black plastic water trough from craigslist) buried a little above level in the ground with several mini water lilies, anacharis, duckweed and mini papyrus, and 2 goldfish to keep the mosquitoes eaten. I have a little waterfall pump with splashing water. It draws dragon flies and toads and there are always some tadpoles. A nice mini ecosystem that can be done inexpensively.

https://aquariumtidings.com/how-to-build-a-pond-a-beginners-guide-to-building-the-perfect-backyard-pond/

People tend to over complicate things. I've had ponds for years, have several of them. None have filters, and only one (actually an above ground pool) has a small solar pump. Some are in close to full shade and others in full sun. Never have any real problems, all it needs is a good selection of plants to balance things out. Very inexpensive and almost maintenance free. I do sometimes measure water temperature (only in the above ground pool as it can heat up a bit), but for all of them it's just the occasional pH, alkalinity and carbonate hardness test. They're done more for curiosity than anything else.

Here are some pictures of two of my small liner ponds in my front yard during construction and done. We do have to cover them every night or raccoons get in them and totally destroy them. They dig up waterlilies and mess with the rocks too.

No fish. No pumps. We use Mosquito Dunks for larvae control. They are full of frogs and dragonflies.

lONG time ponder here - but I remember that overwhelmed feeling all too well. If I had it to do over again, Id start out small like you're thinking - 50 gallons. Get a small 100 gallons per hour (gph) pump + all it needs is small filter over the intake and then create a small waterfall or a bubbling effect out of a pile of rocks (reminiscent of a spring) The circulating water will itself eliminate mosquitoes/larva.

No goldfish, koi, anything in the carp family in this small a pond. These are pooping machines and really are not easy fish at all, in spite of what people might think. Instead, get some rosy red minnows - which are small, naturally short lived fish that will add color and movement, and be size appropriate for a small pond.

Just see how you like it, take your time about researching your next and bigger pond - esp get up to speed re: NITRIFICATION CYCLE. If larger fish+ greater numbers of them you will need filtration that will handle larger bio-load.

Thank you frankielynnsie for that link! I am inexperienced with ponds and recently bought a house with a pond that needs some work and want to learn more so I can take care of it.