I honestly believe that writing or creating your own spell is always more powerful then using one written by someone else. I believe this because you are composing it for a purpose, you are putting your time and effort and research to make it unique and your own.
There are several things that many people use when creating a spell, and this information isn't the only way, there are many ways to create this is just my own experience so please if you are researching and find something that resonate with you and its not what I am saying then go with that. It really is about your interpreting and what you want your spell to consist of.
Some people like to use a spell to coincide with the phases of the moon. This is done for many reasons. I for one mostly work with individuals that have fertility issues, so for me the new moon or waxing moon cycle are more beneficial to me, since the moon is growing in size (hence a baby grows in the womb) But spells like banishment, or removing debt or negative people from your life, those might be better cast then the moon is waning, since you want to ride this and the moon is shrinking in size. Others don't use moon cycles at all so its really your own preference.
Some people like to use crystals. There are so many different types of crystals and each has its own properties and powers. You would need to research which crystal has the properties you want, or in general you can use a quarts crystal for many different things since it is the purest form and covers many areas. Some people have to charge their crystals using salt. I don't use a lot of crystal work, but I bought a nice small wooden box that I charge my crystals in with sea salt. When the moon is full I generally wash them (figuratively) in the full moon by placing them on a windowsill or my balcony ledge. But some people don't use crystals at all.
Candles are very big, and they are used in many spells. They are relatively inexpensive and come in many colors which each represents its own properties. Many spells I have written I use a candle color and the flame is used often in conjunction with some herbs. Most love spells or beauty spells use a pink or red candle. Money spells use green etc.. But you can also use just a plain tea-light which is white, since white is the purest form, and can represent what you are trying to accomplish. Some people don't like using candles, and some spells don't have them in there.
The spell itself generally has several parts to it. You can state words, and I always write my chants prior to developing the actual spell, so it identify what I am trying to accomplish, why it should be granted to me, and finally command it to be done. Many will use "So mote it be" as their final statement, basically ending the chant and closing the request. It is a good phrase to use, but I have also seen other spells that don't use this phrase. I like to have consistency with my spells in having a beginning middle and end and having a closing phrase that you use a lot, helps keep your spells valid to you and I believe this only makes them stronger.
The wording does not need to be long, and it does not have to rhyme, but when I compose a spell I generally like to spend a lot of time writing it and having parts rhyme. Its an art form and I take a long time and put a lot of effort into making sure its linguistically pleasing to the ear and addresses what I am seeking.
Once you have your concept, you can write your chant, figure out when the best time to cast it (moon cycle, or time of day) you can select your candle or crystal, and I always select specific herbs to use, as they all have many properties to them. With my fertility spells I use a lot of parsley and pomegranate. These two have a long history of being used with fertility issues.
The biggest part is the research, finding out what you want to put in your spell and how you want it to go. If you honor or work with a god or goddess you also will want to make sure you have recognized them and have part of your spell dedicated to them, so I work with Ixchel, the Mayan fertility goddess. The Mayans in my research and in speaking with some of the Mexican natives in the Yucatan, always talk about leaving fresh fruit out to honor her. I always leave a sliced piece of fruit for her during my spell and often leave it there for a day and then return it to the earth (compost, bury etc)
This is just some information that can help you get started. Its really a lot of research, and then once you are comfortable and start understanding the properties of herbs, plants, moon cycle, what your specific deity likes, it will become more meaningful as you go. But creating a spell to me is always a labor of love.
I would also recommend joining a coven and seeking out one that can aid you and review your spell prior to you attempting it. This way you will have guidance from more experienced people who can recommend changes or adding things to help make your spell effective.
Hope some of this is a little help to getting started. Again its not the only way and there are many different ways to develop a spell. This is just my own experience.