I seem to have alot of trouble killing any assasin type players, especially Twich, could i have some tips and advice on how to counter these sneaky little runts?

More obvious stuff:

- Don't run around low on health, or even half health. Being near a tower is no guarantee of survival.
- Have an escape option. Some champions have built in escapes (Tristana's rocket jump, Shaco's Deceive, etc.) Often you'll want to supplement this with Flash, Ghost, or Cleanse.
- Get health, armor, or magic resist if necessary against particular champions. Even if you aren't a tank, sometimes you'll have to get defensive items.

Less obvious:
- Exhaust will screw up many (but not all) assassins via blind. Twitch loses a lot of effectiveness if blinded for 3 seconds of his ultimate.
- Silence can work as well.
- Stuns of any kind can often save you.

For any of those, try to avoid using the skill on something else if you fear an assassin popping out. If you are starting a team battle, try to hold off on your stun until you Twitch breaks out of stealth.

Nearly all assassins have low health. Focus them to death with your team.

If the assassin has a single hit ability that hurts a lot, consider getting Banshee's Veil. This works wonders vs. Evelyn, for example. Avoiding her stun weakens her greatly.

John raised good points. Other than that, do your best to keep map awareness. That means:

-Calling out the champions who are missing in action through team chat
-Using wards (stealth-revealing against stealthers, obviously, and the cheaper ones against champions like jungle Fiddlesticks or Udyr)
-Having clairvoyance on the team, to reveal bushes and scout the jungle
-Use champions who can give your team map control through traps (mainly Teemo, partially Shaco and Nidalee)
-Scouting bushes often with spammable skill shots (Mundo's cleaver is the best, but consider others like Karthus' lay waste and Janna's howling gale)
-Have Twisted Fate on the team, to cast destiny when there is an awkward lull in action.