As long as you use your own gods (if any) and meditate or focus enough on your intent the spell should work, assuming it's possible. Don't try making a spell to turn you into a vampire or make you grow wings, that stuff is impossible.
Some good advice has already been given :) My secret to a successful spell is intent and practice. You can perform a spell in your head and have it succeed. Sure, spells bomb, and I know I've had a couple that bombed colourfully, but with anything, practice makes perfect. Don't expect results right away - it took me 10 years to get a service dog! - and don't ask for anything that is physically can't happen, like growing a new arm or reversing time. Or turning yourself into a vampire/mermaid/whatever.
Simple spells are the best ones to practice with. Try and make some that can be as simple as possible, then once you've feel like your ready, then try to progress slowly, beginning with candle magic and then moving to things that require a lot more materials etc. Make sure you know the basic properties of magick, and just the basics in general.