I would be more than happy to share some advice with you!
The best way to start is look at all the creatures we have here on Earth! It's also good to look at how animals have changed over time to adapt to their surroundings. Aliens would be the same way!
How I create my own species, is to use different mixtures of the animals we have, even mythical creatures, and alter it just a bit so they look more unique. Sometimes giving them more than two eyes (or just one... or even no eyes! Maybe they'll rely more on hearing!) is a good way to start. You can even make up your own parts to add to them!
I hope this helps!! C: