This is my first tab, I'm pretty sure that it is 100% correct but you can be the judge of that :)

Standard Tuning
No Capo

Verses are all the same.

E|---7---------7---------7---------7---------|
B|-----9---9-----9---9-----9---9-----9---9---|      \
G|-------9---------9---------8---------8-----|       \
D|-------------------------------------------|        \     
A|-------------------------------------------|         \
E|-------------------------------------------|          \
                                                          2x
E|---9---------9---------9-----------9-----------|      /
B|-----9---9-----9---9-----10---10-----10---10---|     /
G|-------9---------9----------9-----------9------|    / 
D|-----------------------------------------------|   /
A|-----------------------------------------------|  /
E|-----------------------------------------------|


Choruses are obviously all the same.

 |          E          ||        Abm         |
E|---7---------7---------7---------7---------|
B|-----9---9-----9---9-----9---9-----9---9---|     
G|-------9---------9---------8---------8-----|      
D|-------------------------------------------|           
A|-------------------------------------------|        
E|-------------------------------------------|         
  
 |         C#m         ||         A              |         
E|---9---------9---------9-----------9-----------|     
B|-----9---9-----9---9-----10---10-----10---10---|   
G|-------9---------9----------9-----------9------| 
D|-----------------------------------------------|  
A|-----------------------------------------------| 
E|-----------------------------------------------|


Bridge.

B                              C#m        A
  All his money can't buy me a time machine no
B                          C#m         A
  Can't replace you with a million rings no
B                             C#m       A
  I shoulda told you what you meant to me woah
B
  Cause now I pay the price...



Until end of song...

 |          E          ||        Abm         |
E|---7---------7---------7---------7---------|
B|-----9---9-----9---9-----9---9-----9---9---|     
G|-------9---------9---------8---------8-----|      
D|-------------------------------------------|            
A|-------------------------------------------|        
E|-------------------------------------------|          

 |         C#m         ||         A              |         
E|---9---------9---------9-----------9-----------|     
B|-----9---9-----9---9-----10---10-----10---10---|   
G|-------9---------9----------9-----------9------|   
D|-----------------------------------------------|  
A|-----------------------------------------------| 
E|-----------------------------------------------|


Hope you liked it :) I will probably be putting up some other Katy Perry songs.
If you have any requests please tell me.