Hi Momo and welcome to the forum. I"m really sorry your betta is so sick.
Right now, it looks like your boy has some serious fin rot. First, do a partial water change of 25% right now. Tomorrow, do a 50% change. The day after tomorrow and every day after, do a 100% change. One of your best keys to fighting this fin rot is clean water.
Momo is going to need some medications. Your best bet is Seachem Kanaplex, a broad spectrum antibiotic. You will most likely have to order this online but you can also check any local tropical fish stores near you. If you don't want to wait for this to ship, then go to your nearest Petsmart and get Maracyn I and Maracyn II. You will use both of these to treat your fish.
Once you answer the sticky and I know your tank size, I can help you with dosing.
In the meantime, keep your betta as warm as possible.