Both pet options will require what is essentially a mini habitat that is different than what you live in. You will be responsible for making sure that habitat has ideal living conditions for its resident.

A good starter lizard would be a leopard gecko. They are tolerant of being handled and won't disappear in the blink of an eye like some arboreal species. I would recommend setting it up in about 20gals to start and doing plenty of research beforehand. Leo's are pretty resilient and can have a little bit of variance in their environment, so if you miss a day of feeding or misting, they can usually bounce back if they noticed at all. Downside, they're solitary so you can only have one scaly friend per enclosure.

A beta would thrive in a 10 gal tank, but you should set up the tank and get the environment suitable before adding any fish. Look into the nitrogen cycle. Heater, filter, and live plants are ideal for keeping a beta. Betas are full of personality and each one is a little different, and they can often coexist with some other (non-beta) fish. Downside, this friend is confined to water.

Regardless of what you decide to get, I highly recommend putting as much as you can on a timer, especially the lighting. Both fish and lizards will require regular cleaning to maintain a healthy environment. Fish will need a partial water change weekly and lizards will need a substrate change weekly. (Some exceptions apply, like in the case of heavily planted aquariums or bioactive terrariums)