"As a cis person, it is hard to be able to relate to trans people and their struggles, especially when other parts of the cis population are against the trans community. But, it is important to learn to support the trans community." -anonymous
Stand up for your peers. If you see someone getting bullied, don't be afraid to educate them and tell them why what they're doing is wrong. You should always stand up for your friends, but especially in situations where the person is vulnerable or likely to stay quiet.
The best way to show support is to treat transgender people like you would treat anyone else. You don't have to be extra nice to them because you feel bad, and you don't have to treat them like they are special, this will make them stand out and feel isolated.
Avoid back handed complements. Don't say "I never would have known you're actually a girl" when referring to a trans man. If they're a man, then they were never a girl. Instead, give normal compliments like "Hey, I really like your hair."
Try not to make Transgender people uncomfortable by asking them personal questions. Their name or sex assigned at birth is none of your business. You should not tease them about it, if they don't want to tell you then they won't. Don't try to convince them to tell you, as that can be very uncomfortable for them to hear and talk about.