Think you're capable of casting a spell or two? Would your powers allow it? Well, here is the general guideline for casting a spell.
Contrary to popular belief, not all spells have to rhyme! It's not particularly the words that decide whether your magic will work, but the power behind them. If you are powerful enough, then it shouldn't matter too much if you don't rhyme. It can be as long or as short as you like, but don't go overboard!
Now most likely your spells will be asking for something specific, that your magical, physical powers don't particularly lend to. Make sure you state at some level what exactly it is you're attempting the spell to achieve. In the following example, see if you can work out what the spell is for.
"Dark is here and sight is dim
Though sun will never rise on whim
Take my power, three times three
And bring the daylight, mote it be."
If you answered correctly, you would've noticed the part where it says "bring the daylight" and also the other clues in the spell. It's clearly a spell to turn the night into the day. Although it rhymes, it's not necessary, nor is the "three times three" or "mote it be" sections, which tend to crop up in traditional Wiccan spells.
Now onto the technicalities of spellcraft.
You've cast your spell, and now don't know what exactly follows. Well, on no account do you ever decide whether the spell works, or what the result is. That is up to The Powers That Be. The best you can do is only hope for the best. Whether your spell backfires, works smoothly, or does nothing at all, you must wait for The Powers to post the result, and not make it up yourself. Your answer may not come immediately, or if the spell has not worked, perhaps not at all, but as long as you don't determine the outcome yourself, you will be casting your spell correctly.
Don't fret if your spell doesn't work, as you become more active in the World, your powers will grow, and your spells more likely to succeed. Keep trying - but don't overuse the spell-casting option, or you might find The Powers becoming somewhat angry with your lack of initiative!
Good luck, and happy spelling!