i think you need to clarify by what you mean, "write my own song". do you mean writing just lyrics or just a melody or both.
neither of them is a simple process. although, one begats the other. and now to muddy up the waters, there is no singular way to write a song. there is no set method of writing a song. there is no rule that says to write the music and then the lyric or write the lyric and then the music. forget the stigma of rules for there are none.
first, as has been mentioned elsewhere, writing lyrics is a difficult thing. but let me digress a moment. earlier i said "forget the stigma of rules". let me re-phrase that to be, "forget about the stigma of rules except for this one .... -> rewrite". if there is any rule to follow, "rewrite" is the rule. no matter how well you think you have written a lyric, it can and will be better by rewriting. by finding new ways to say that "same old cliche". by finding ways to put "color" into your lyric.
so, how to get those writing juices to flow. in other words, "bring the muse in from the cold" ...... i know, i know, this is going to sound silly, stupid and anal. get a note pad, a journal book with blank pages or something you can carry with you day and night. when something strikes your fancy, write it down. no matter what time of day or night, write it down. you may hear someone say a word that brings up a memory of the smell of pancakes your dad used to make. write it down. a light may flash that hurts your eyes, "and in a flash of light" .... write it down.
at this point, don't worry about meter or rhymes or even spelling, just get it down. a little thing i do when I take the Mrs. grocery shopping and i'm sitting in the car for what seems like eternity; i just write down random words. doesn't matter, random is good. and then 45 hours later when she comes out, i've got a nice list of words that i can keep to jog my mind when i sit down to write.
a lot of folks say "make a title" and then write a lyric around the title. not a bad idea. but you need to remember, the title is, in most cases, the hook to the lyric. the hook is what "hooks" the listener and makes them to continue to listen to the song. "hook, line and sinker".
some folks start playing a chord progression on their guitar, a simple GCD. and then they mix is up by adding an Am or a Bb and that triggers the writing process. in other words, you start a chord set, have a rhythm cooking and while you're playing you just start humming a melody to the chords. now you've got a base melody and certain words lend themselves to a melody or rhythm. pretty soon you have the beginnings of a lyric.
i know i've rambled on a bit, but these are just some suggestions to get you started.
I keep these at my "writing desk" at all times, a rhyming dictionary, a thesaurus and a book of quotations (like Bartlett's Book Of Quotations). Get a set and keep them handy. You would be amazed at the words and phrases you can come up with by having these three tools at hand.
one last note on words. the English language is incredibly large. one source counts English words to about 850,000, give or take a few. the French language has, by contrast, less than 100,000 words. if i remember correctly, the average English speaking person uses less than 10% of the English language in a lifetime. John F Kennedy's command of the English language was in the neighborhood of 45%. (please correct me if i'm wrong)
Well, thats, a start .....thanks for getting this far.
Best to you,
Les