To see how many points you need to level up, visit the Levels page.
Every time you feed an animal (by clicking the apple icon hovering above it) you gain experience points. How much an animal produces depends on its recharge time. Here's the exact amount of Exp you can collect from each completed animal family:
A single animal earns 50% of the total, a pair of animals earn 62.5%, and three animals earn 75%. For any decimals, round down to the nearest whole number (for example, if you calculate that your animal family with two members will earn 18.75 , it counts as 18 ).
Which animals are best for your village will depend on how often you play the game. If you play for long periods of time, it might be best to have animals that recharge quickly. But if you only check your village once a day, then animals that recharge over longer time periods are probably best for you.
For example, the Sloth earns 2 every 30 seconds, and the Sneaky earns 320 every 48 hours. If you play the game for an hour continuously each day, feeding the animals whenever possible, you will earn 640 over the course of two days. During that time, you'll only earn 320 from the Sneaky (and that depends on whether it came time to collect during the two hours you were playing). So if that's how you play, the Sloth is a very good earner for you. However, most people do not want to have to feed animals every 30 seconds, so those that collect at a slower rate are good choices. Let's say you just check your village once a day, and you don't play for very long when you do. For that player, the Sloth will only earn 4 points over two days (because they'll only collect from it twice, once each time they visit their village), but the Sneaky will still earn 320 during that period of time. Basically, remember that animals with shorter recharge times can theoretically earn much more than those with slower ones, but it requires much more work from the player.