Hi there weisong. Welcome to SRC.
You seem fairly new to the reef keeping hobby. Perhaps you might wanna read through some of the pinned articles posted here.
After you have a thorough understanding of the basic requirements of reef keeping, you can turn your focus on your initial plan, to keep a pico tank.
Keeping a small tank has its pros and cons, and by understanding its limitations, it will help you decide the size of the tank you can keep.
A budget of $500 is plenty for a basic setup. If you can take a photo of your computer desk and share it with us here, it would better help us in giving proper advice as to whether your desk could take the weight of the tank or not. In general, computer desks are not that sturdy, and do note that salt water has a higher density, which adds to the weight of the tank.
Computer fans are not that powerful, so if they are insufficient to cool your tank, you can consider getting an Ice-Probe. Do take note of the rate of evaporation if you use a fan.
Feel free to post questions if you need more help, but mostly, answers can be easily found online.
Cheers, and happy reefing!