Start slow man, when Im getting back into training I do walk/run between telegraph poles, slow run and brisk walking, try throwing in some push ups and sit ups in before the walking phase, aim for at least 10 x 10 sets of each over the time your out, you want to keep it up for 45minutes at least for cardio and a max time of 45 minutes for weight training.
As for your heart rate, a good test is that if you can have a conversation, albeit with some huffing and puffing, you are going OK, BUT, if you cannot get your words out back off a bit.
Always carry a water bottle to, keep away from any energy drinks as they are full of sugar and salt and if you have a good diet you do not need it.
After you have eaten dont train for at least 1 hr minimum but have some good carbs within 1/2 a hour of training.
Most important thing is to start slow and work up your intensity, first month should be taken easy and always record your times and achievements, ie 5klm walk run in 38min with 10 x 10 push ups, 8 x 10 sit ups, I do this style on Mon Wend Fri with squats and deadlifts on Tue and bench and pulldowns on Thurs, again recording your reps and weight used .
There is 11ty million different fitness programs you can do, this is what I find works for me after a spell.
I do 10 x 10 on all weight exersices as I fell this works well for me, remember no more than 45min training.
Dont waste your money on the Protien powders and pills and stuff they sell, good food is better and costs less.