The following advice hold pretty much for all twins.
1. One vs. many (defensive) will be VERY difficult for you. The 38 is a big target and even a two-weeker number guy will find its mark. Keep your SA up and know when to leave, or fly with wingmen.
2. Never ever allow a shot on your full plane. Learn to roll just before the other guy is about to fire even if it puts you in a less favorable position. The P-38 is paper-thin when viewed from the side, so break and if you cannot get completely out of the way, give them your side.
3. Once someone saddles up on you, it will be very difficult to wiggle your way out of that, like yaks, 109s or even P51s can. Do not let anyone get on your 6 - turn into them early, 1.5k with a minus sign is already too late. SA must be much higher than with other planes and the "threat range" much larger. As Delirium said, they are all looking at you.
Of all the twins, the P-38 has the best survival tools. It may fall short of the mossie deck speed, but your max speed goes linearly up with altitude. A few kft up, you are already in a better ranking speed-wise. Above 10k you are getting into your element and both your speed and climb start to get impressive. Above 20k you'll be alone, so come back down.
If you are in a learning phase, forget about smart flying and forget about your score this tour - go wild. Fly into the horde at low altitude and turn with every Brewster you see. That is the only way to learn how to handle a plane. After a tour of that, start to think about tactical flying.