Hello Rox...
If you're a beginner, I'd recommend a hardy fish species that doesn't mind less than perfect water conditions, which is what happens to those new to the water keeping hobby. Some of the live bearing fish like Guppies, Platys and Swordtails are good. If you prefer egglayers, then White Clouds, Rosy Reds and Red Eyed Tetras. You have a medium sized tank, so cool water Goldfish would work too. They don't require a tank heater. I generally keep one fish for the first 15 to 20 gallons of tank volume and then 5 gallons for every fish after this. So, you could keep around 6 in a 45 G tank. These fish will grow much larger, but this takes many months. So, you'll have time to consider a larger tank for these fish. Read up on the nitrogen cycle before you start and once the tank is cycled, you should remove and replace half the water every week to maintain suitable water conditions for the fish.
Have fun,
B