Actually, i respectfully disagree. A beginner mage has no business writing their own spells. They probably wouldn't even have the slightest idea where to start (hence the freqent questions of this kind on the forum): what herbs to chose, basic candle techniques, energy manipulations, helpful correspondences, to name just a few.
My advice: Spend years doing spells that experienced workers have created. they have gone before you and paved the way. Do spell kits. Learn the basics. Then, as a natural progression, you'll be able to create your own spells without much effort and confusion - it will just naturally flow out of you.
I think possibly THE number one "Modern Spell-Casting Myth" is the falsehood that spells you've written yourself are more powerful than those written by another. This is actually laughable, but it's a myth that's perpetuated a hundred times a week on this site alone.
To answer your question: start out with magical techniques that aren't really spells, per se, like grounding and centering, focusing your mind, creative visualization, creating sacred space. Learn other techniques such as cleansing and spiritual bathing. Good beginner spells include those for blessings and success. There are a few covens on this site specifically for teaching the basics (actually teaching).
On the topic of creating your own spells, here's a rant that i originally posed in the Practical Witches Coven under tips for writing your own spells. I'd recommend reading the entire thread if you're interested.
"Personally, and i don't mean to be controversial, but i think writing your own spells is a bit over-rated. It's kind of like making your own soap or brewing your own beer. Some people are interested in learning how to do so; others aren't. Store-bought soap and beer are just as good - and maybe better - than the home-made stuff. Same goes with magic.
"A spell some random person writes in ten minutes is not going to be as good as an old school, traditional spell - let's say the Trinka Five, or the Popcicle Stick Reconciliation Spell, or how about the Abracadabra triangle. There's a reason the same spells or the same family of spells are used for centuries at a time. It's because they are effective and get results.
Here's such a superb article written by one of my collegues, Mike Rock, on the subject. I highly recommend anyone interested reads it. The article has a little bit of an edge to it since he was writting it in response to a point of view he was defending, but it totally gives a lesser-given opinion on the subject of the importance of writing your own spell:
But your question is a very valid one, and every practitioner gets to a point where they want to write their own spells to best address their unique set of circumstances.
Before you get to this point, though, let's assume you've cast 50 or more spells over the course of a few years. You should know 20 ways to dress a candle, how to attract things into your life, how to repel things, how to use personal concerns, herb correspondences, use of stones and minerals, cleansings, healings, raising and directing energy, creative visualization, and the like. You've used magic for a diverse set of circumstances.
You should then have a body of techniques that you've developed from performing spells these that you've gotten out of books. All people should start off with spells from out of reputable spellbooks or spell kits.
If you don't yet have a body of techniques from which to draw from, then you should probably do more spells out of books, try spells that utilize different techniques, and purchase reputable spell kits that might implement different supplies as well. As you can see from this site, the spells that newbies write oftentimes aren't very good at all; that's what happens when inexperienced people write spells.
I do think people are greatly disserved from one of the modern magical myths that spells we create ourselves are stronger than those we don't create. I think it's actually the other way around, usually. Still there comes a point in time when every practitioner should be able to do it"... (cut for relevance)