I'm taking cephalexin will it affect it if i drink one night but don't drink enough to where i get drunk?

No alcohol. : You should not drink alcohol anyway.! And not with antibiotic!! But you know that already! Yes?

It is fine: Cephalexin does not interact with alcohol. Obviously excessive drinking is not healthy for you but 1-2 drinks per day are perfectly fine for an adult. Do note that female can only tolerate 1/2 the amount of alcohol vs male. I also note that you are also underage for drinking alcohol.

Antibiotic + alcohol: The idea that alcohol and antibiotics have some terrible interaction is mostly a myth. There are a few antibiotics which would interact badly with alcohol (metronidazole, trimethoprim/sulfamethoxazole for example) but for most antibiotics a modest amount of alcohol is not an issue. (See http://theconversation.com/mondays-medical-myth-you-cant-mix-antibiotics-with-alcohol-4407 )