There are several ways to become a happy owner of a tricycle. Some of them are simple, others are more complicated and risky. I will list some of these possibilities here, and guarantee that if you try them all, you will be able to get your parents to buy you one!
1.
If you're lucky, you have parents that think that learning to ride a tricycle is an important developmental milestone, and so in due time, they will, without any outside stimulation, go and buy you a tricycle. I thought I was that lucky, but somehow my parents never got around to buying me one, despite several declaration in that sense, and so that tricycle never showed up.
2.
You can ask your parents directly to have a tricycle. But if your vocabulary is limited as is often the case for children of the tricycle-age, it may be difficult to get them to understand what you want. In my case,
BrrrNaNa Odda (in a simple translation: Could you please be so kind to give a bike to Oscar), did not work on my mentally retarded parents. Despite me repeating this a million times, they never seemed to understand what I meant!
3.
You can become a big bike and motorbike fan and hope that your parents indulge you and buy a tricycle to further stimulate your passion for moving objects. In my case my parents indulged my passion by buying me cheap, tiny, bad-quality motorbike replica that have had to be replaced 2 times already because they fall a part when the demon on my shoulder has encouraged me to throw it on the floor!
4.
Every time you go to the playground and there are children there that bring their tricycle and bicycles, you should get to try them. Once you're on the bike you should go straight home with it. In my case, every time I sat on some other kid's tricycle and said to my mum
heim, heim (home), she lacked the courage to help me bringing it home, instead she just helped me going around the playground on it. Coward!

5.
You can make your own tricycle, using any moving object in your possession, hoping to send a signal to your parents that it's time to buy that much desired tricycle. In my case I used a train that has the function of toddler walker (you know, the thing that helps you learn to walk), and sat on it, as if it was a tricycle (as you can see on this picture). But my parents just thought, why buy a tricycle when Oscar found a way to make his own using his fantasy!
6.
This adivece is the same as above, but with a little modification to help in case your parents are as mentally retarded, and as slow to understand things as mine are. You make your own tricycle, using any moving object in your possession, and then you have a minor accident (and here lies the difficulty, you wouldn't want to have a major accident would you?) with it, and so send the signal to your parents that despite the great fantasy of your child, that can make a tricycle out of almost anything, it's now time that they buy you one! In my case it took a graceful front-fall right on the spot where the picture above was taken (and only a few minutes later actually), a broken tooth and a lot of blood coming from wounded lips, to open my parents eyes. THEN, finally, they went to buy me that much desired tricycle (and the train disappeared misteriously at the same time). Now I'm the happiest boy under the sun.

Next I'll give you some advice on how to get your parents to buy you a horse...this is a project that I'm still working on!