Becoming pregnant takes time, patience and planning. The first thing to be aware of is that some couples can take up to a year to conceive so do not worry if it seems to take a long time.
The time to have intercourse is around the ovulation (egg release) phase. This occurs for most women about day 14 of their cycle. Day one is considered to be the first day of the period.
So having intercourse regularly for about 5 days around this date increases the chance of pregnancy. Also using a thermometer or ovulation kit can more closely map when you are fertile.
Do ensure that you are close to your ideal weight, i.e. not overweight or underweight and start taking 400microgram of folic acid daily as this is an important vitamin for development of the embryo.
Getting stressed about ttc, can in itself lead to periods becoming erratic. So although you do need to track your periods, I would suggest that you continue having intercourse when you like, have fun - do not make it a chore, and try to forget about the underlying plan. Having a more relaxed approach should result in pregnancy at some point in the next few months. If your periods were to continue to be irregular then some tests can be done then but this is not needed just yet.