Fairly straight forward really, just rush ahead of your teammates and try and get kills before them. Knifing gets you bonus points so try that. However, dont be too rambo as you will end up dying and needing to be revived. Losing you points and give your opponents points for reviving you.
If you arnt too good at that, try and find the Death Card in the level. Doing so will automatically give you 2000 points. Find it in single player first, know where it is, then when you play online you can try and grab it first.