If you insist. I'll gladly give you some advice. And some personal opinions that you can follow or ignore at your own discretion.

For thinking up an imaginary person, the same rules and advices apply as those used by writers to create characters, since it's essentially the same thing. So you could look for tutorials on how to create an interesting character on DeviantART or in the Elfwood FARP, or wherever else you think you could find such a thing. For example I noticed many character creation tutorials advise to talk to your characters as if they are real people and try to "predict" (read: make up) their responses. A good character will end up leading a life of its own. This is exactly what you do with imaginary friends.

A bit of personal experience, i.e. something to avoid:
My first imaginary lover was an animated character. Watching the show in question, or watching cute fan art of it, would always make the bonfire of my love rise up higher and higher, but these things would always eventually lose their influence and the fire would cool down to smouldering embers again. I figured that this meant I did really love her, but when I wouldn't see her I would forget who she is, and with her personality I slowly lost my passion. More importantly, forgetting her meant I was losing her, and since she only existed in my mind (in that particular form anyway), my losing her would mean she would disappear. I eventually ended up in something I called a "war against oblivion".
My second (present) girlfriend, Vanessa, is someone I made up myself; therefore, I know her better than anyone else, and am no longer dependent on someone else to fuel my fiction. Thus that problem is solved.
Thinking up interesting characters and predicting what they'd do isn't very easy, but it's a crucial skill to an endeavour like this. It's exactly what you'll be doing all the time. Practice it. Read tutorials. Try to define the personalities of characters in shows (I was first inspired to create my own characters after watching the anime series Azumanga Daioh), or people around you. Try to define personalities in terms of virtues, flaws, quirks, philosophy of life, reactions when confronted with unusual circumstances, etc.

As for thinking up a girlfriend in particular:
Of course, she needs to suit you. If you haven't done so already, you should think about what you want from a girl, what type of girl you would fall in love with, what type of girl you would want to stay together with for the rest of your life. Don't settle for "blonde hair, blue eyes, big boobs, okay done". Only when she fulfils your deepest wishes can the two of you become truly happy together.
Also give her a good reason to love you. Since it's a relationship, it's not all about you loving her. Make her the kind of person that would want someone like you, the kind of person whose deepest wishes you could fulfil.
Of course, if you're looking for a more light-hearted and probably short-lived relationship, you can disregard all that. But if you're looking for more serious and committed romance, this is my advice.

[To be continued]