Hi
Sexual thoughts are natural. Our body (with neurological systems and chemical changes) and mind are inter related. When there is a change in our body, it certainly projects in mind also. Vice versa, if mind is influenced, then we can see certain changes in our body. It is quite a simple mechanism.
Coming to your question with reference to above fact. You can get sexual thoughts or fantasizing thinking flow only when your
testosterone is produced inside your body and semen accumulation. Now, without any warning, you will get sexual feelings (from your own body) and start thinking in N number of multiple way (fantasy). But in fact sexual behaviour is for reproduction purpose only. All those ideas and fantasies will slow down once you realize that sex is an out come of reproduction purpose.
Just like when your stomach is hungry, you will start fantasizing about very nice foods. You might think about Biryani, samosa, or nice andhra meals or pizza or anything which you feel satisfies your hungry. Like wise, when vital chemicals are produced, you feel sexual drive, which further allows you to think or imagine anything to satisfy the drive.
During hungry, if you have samosa or pizza, it's not good for your health. But you can have good meals or breakfast items which will satisfy your hunger at the same time keeps you healthy.
So dear, when you get such drives, you can imagine how ever in comes to your mind. But try to channalize towards your natural counterpart gender. But not objects.
In another way, you get aroused to an object as it reminds you of your imaginations about counterpart gender.
These imaginations are within you only. If you attempt to bring it as a behaviour, then please you can convey to your parents, or good friends or psychologists. So that, they will guide to in the right direction to let you have a healthy life.
Next Steps
Reduce, spicy and high junk foods. Try to avoid, more garlic and fried foods. Eat lots of fruits. go for a walking. avoid friends who talk fantasy about this matter. tryst friends who look about it in a healthy way
Health Tips
There are so many and many other things in our life. sex is just not even 1%. Focus on your studies and treat your counterpart gender very gently and protect them. the nature will give you the wisdom of handling your hormones. brave boy you are... all the best