How to step up from 3.3v to 24v?
I would suggest you find some power source and use an npn transistor to switch your relay.
Imagine the relay needs 100mA then you would have to provide some 720mA from your source!

Better to switch as following

V+ -------------------|
                      |--------------
                     |  |           _
                     |  | Relay     ^  Diode
                     |  |           |
                      |             |
                      |-------------
                     /
serial ----- [R1]---| Transistor
          |          \
         |R|          |
         |2|          |
          |           |
Gnd ------------------|


Calulate the resistors according to the Hfe of your transistor