well first off [i'm sure you know this but it still must be said i'm afraid] pregnancy spells help increase the chances, and can help with having a safe pregnancy. these spells however won't make you instantly pregnant. you need to actually have intercourse with someone to fertilize the egg.
it is wise to speak with your doctor first so you can make sure there is no medical issues on either the mother or fathers side preventing conception.
while i don't know a spell off the top of my head you could try drinking raspberry leaf tea, before [and while] drinking the tea, infuse the tea with some energy, and visualize your desired outcome. [you could also say a chant to a goddess of motherhood and/or fertility for help such as Aphrodite, Bridit, Freyja, Isis, Min, Tawaret, and Venus to name a few]
with all this said, i noticed on your profile you're 17, are you sure you want to get pregnant now? are you stable enough to have a child [financially, relationship, education, maturity] i personally don't have a problem with young mothers, i have many friends who had their kids at 17, but because of this i also know the struggle. the child will come first, you will stop going out with your friends [unless your boyfriend watches the kid, or your friends agree to hanging at your house] does your family support you because you will need help from them. many of my friends were lucky and their parents helped a lot, but i did have one friend whose parents abandoned her at 18. they boarded up their house while she was away with her son and left a note saying she needed to take responsibility now that she graduated high school. just like that she needed to quit school, sign up for welfare to get first and last rent, find a place to live, and raise her 1 year old alone.
like i said i know many young parents, you won't love your child any less, and the young parents i know don't do any worse than parents in their late 20's, it's just by waiting you've grounded yourself and are a little more secure. if you haven't already i suggest talking with your parents. [don't get mad, just my two cents] good luck to you.