Your best chances of conceiving are when you ovulate. Here is an ovulation calendar for you :
American Pregnancy Association : Getting Pregnant: Ovulation Calendar
However, knowing when you ovulate could be a little tricky if your period is going through irregularity. If you are irregular, it might be best to buy an ovulation kit, they are much more accurate.
It can take healthy couples up to a year to conceive, so don't lose hope. It will happen for you eventually, but it may take time. I know some people that have tried to conceive for years and have still had no luck. If you don't conceive within a year, I would go to the doctor and have your fertility checked, and have your boyfriend get his sperm count checked as well. Sometimes it takes just one time, and sometimes it seems like it takes forever.
Also, here are some hopefully helpful tips to aid you in getting pregnant :
Tips on Getting Pregnant
I would also start taking some prenatal vitamins if you want to become pregnant. It won't help you get pregnant, but it will give your body vitamins for when you do get pregnant. :)