For muscle definition you need to look at doing high reps and low weights. So you can be very simple and do things like press ups, sit ups, planks, dips etc.... Running and cardio workouts do help tone muscles up a little, but will also cut down the at on our body which will show of the muscles that you have better.
If you have the time and money you can use a gym. If you start looking at wanting to gain more
Muscle then alter the weights so it's a heavier weight and the reps will go down.
Maybe look at altering you diet to get more protein in your system, so things like chicken, oily fish, all meats really, but they have a high fat content also, so you could look at nutrient shakes.
