You seem to understand the technical requirements of a sonnet: abab rhyme scheme, 10 syllables in a line, 14 lines. There are also "requirements" regarding the content of a sonnet.
Although sonnets are a form of poetry, they are not about free-flowing emotions. Sonnets usually are formed according to a logical structure.
The first 12 lines discuss a situation or problem; the last 2 lines offer a solution or conclusion.
So, the first thing you must do is to choose a topic that lends itself to this kind of structure.
Topics such as, "How much I love ice-cream," or "My boyfriend is the most handsome guy in our school," are not very well suited for a sonnet, because there is no "tension" between problem and solution, or situation and conclusion.
A better choice might be something like:
a) I love ice-cream but it's too fattening and it's giving me pimples; or
b) My boyfriend is handsome, but he's lazy and abusive.
With topics like these, you can develop your complex thoughts in the first 10 lines, and then come to some sort of conclusion in the last 2 lines.
See the link below for some excellent tips on sonnet-writing.