The answer to 'how to fix sink' is to first identify the issue with the sink, such as a clogged drain, leaky faucet, or damaged sink basin. Depending on the problem, you may be able to fix it yourself with some basic plumbing tools and supplies. However, for more complex issues, it's best to call a professional plumber.
If the sink is clogged, you can try using a plunger or a sink snake to clear the blockage. Start by removing any standing water from the sink, then place the plunger over the drain and plunge up and down several times. If that doesn't work, use a sink snake to reach deeper into the drain and dislodge the clog. For leaky faucets, you may need to replace the washers or other internal parts. If the sink basin is cracked or damaged, you may need to replace the entire sink.