FitVandaag - I want to gain weight!
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But I can't.

Most proberly dream of this, but some do have this problem or think they have. The solution is often always simple, you don't eat enough. You simply have to train yourself and especialy you stomach to proces more food. And that food should be rich in energy. Liters of fat free yoghurt or big salades won't do the job.
Try to eat energy rich foods, don't eat lean products but choose the more fat variances, and talking about fat, eat more fat!

Before I start a whole story, let's keep it simple. Eat at least 3 meals a day, if you don't start doing so. And besides that, take at least 3 times a day something extra inbetween.
Start with this and do that for a week and weigh yourself and look what it brought you. Did you not gain weight, eat more every eatingmoment the next week, after a week you again weigh yourself and look at the result. Keep doing this until you see your weight rising. At that point your don't increese your foodintake anymore, but keep eating what you do. 
It won't be easy, but keep at it, it is a matter of training your stomach, so you have to eat!
Next to eating being very active plays a role, if you are very active and use a lot of energy this way, try to lower that, try to avoid any unnecessary exercise. Of course you shouldn't exaggerate and only lay on the couch. But walking, cycling or standing for hours a day should be avoided, if possible.
For athletes who want to gain weight, often being fitness and bodybuilding people, there are the socalled hardgainers, in general what I wrote above does also apply to them to, they simply don't eat enough. And on top of that they proberly also train to much and do to many sets and reps in their training.
Training means giving your body an impuls and after that its needs time and buildingblocks (enough and the right foods) to process that trainingimpuls.
And by the way, real hardgainers are rare, most who think they are, have no patience and have the wrong expectations, without doping you simply can't build mass very rapidly. What we see in fitness magazines are often users and the stories written next to the pics are mostly made up nonsens. Pro bodybuilding is knowing how to use the right doping, so as a non user don't try to follow that. Be realistic.
Trainingwise, forget about isolation exercises, focus on the base: squats, deadlifts, benchpress, rows, shoulderspresses. Do exercises at which you use more than 1 joint. After a good warm up do a few heavy sets, that is enough. Keep the reps low, between 5 and 10 max.

Cardio is fine, but keep it short and don't do it to often. Do it for fitness reasons.
Foodwise, eat enough and especialy enough proteine, to grow you body needs aminoassits, take during every eatingmoment a good proteine sorch. Eat at least 2 grams proteine per kg of bodyweight.