Yo have ilio tibial band strain (ITB strain). It is VERY VERY common in new runners and it's caused by tightness of the iliotibial band, which runs down the outside of your thigh from your hip and round your knee. It can be made worse by poor running technique and bad shoes. You need shoes designed for your running gait, foot shape and style - it's best to get them fitted at a running shoe store, i know it's expensive but so are operations on the lower limbs to und damage caused by running in crappy shoes! You can treat it with rest and stretches - you need to do ITB stretches 2x a day whether you ran or not.

Here are a bunch of stretches. Don't lose heart, this is super common and it WILL resolve. Until the pain is gone walk all of your programmes (so you're walking 5km instead of walking/running). Rest really helps. I know it's disheartening, but running takes a lot out on your body, and even the most professional hardcore runners have to rest when injured. So even though it might feel like you just started and now you're resting already, that's kind of how running is...

Bx