How long is your cycle usually? It would be reasonable to do a pregnancy test four to five weeks after your period started.
Unless you are tracking your ovulation you won't know exactly when the best time is but for most women it is between day 10 and day 18 of your cycle so try to have sex a few times in that week. If you have sex three times every week except your period you will be likely to have fresh sperm in your tubes when they are needed. However if you have a two year old it is sometimes tricky to find the time for sex three times a week!