Yes, you can write your own spells. I think they are more powerful because you have the need and intent behind it for doing the spell. It also puts more of yourself in it. Use what you need or want to put your intent out there.
Here is a good thread written a while back. Remember to take out any spaces in the link:
materials can make it very powerful. the supplies just have to correspond to the spell. for example, for a spell to attract love, you might want red pink candles, red pink paper, etc. colors are important, but white can be used for anything. herbs and crystals help a lot too!
Yes you can write it and add materials, but materials aren't necesarry. It's all about your energy, so in case you believe you can focus it yourself with no materials, that's the way you should do, that I also find easier and better, because it's just simple and effective.
Yes. You can make your own spells. You ahve to think about what you want for the spell. When you find herbs and runes and other items, you need to look at meanings and colors that represent what you want or summon. For example if you did earth magic you would use a green candle. Water woud be blue. Fire would be red or orange. Air would be white or yellow. Spirit is purple. And it also helps if you mention a god/goddess in your spell. Like if you want rain you would light a blue and green candle, say your incantation, and pray to a goddess of rain or something. If you invoke air it would be helpful if you had a charged wand. Because it represents the extension of the arm and would therefore, help invoke air. :):):) I hope this helped a little bit. :)
Just do lots of research and herbs really help In spells, it having releasing it's energy for your cause (though you would want to be nice and ask the plants permission before picking it) and if you haven't already, study energy manipulation.