"How will I know what to do during the service?"
Don't worry at all. When you come into the sanctuary, an Usher will greet you and give you a bulletin, which contains the service order for the day. You can view the latest bulletin here.
Most of the time, if you're supposed to stand up, or sit back down, the pastor will say so (additionally, if there's an asterisk * next to something in the bulletin, it means to stand if you're able). Our services might include some call-and-response portions, too. In all of those circumstances, the part said by the pastor or the liturgist (reader) is printed in normal text, and the congregational response is in bold type.
At the end of the Prayers of the People, the pastor leads us into the Lord's Prayer. If you haven't memorized it, no worries, you can find it in the front pages of the pew hymnals. Presbyterians typically say "forgive us our debts as we forgive our debtors." But, if you grew up saying "forgive us our trespasses as we forgive those who trespass against us," or "forgive us our sins as we forgive those who sin against us," say either one. Pray as you are comfortable.