Thanks for asking such an important question. After all, prayer is an important part of our Christian life because it helps keep us focused and oriented toward God.
As for how often we should pray, Jesus, in instructing us to pray "give us this day our daily bread" (Matthew 6:11), teaches us that we should be praying every day. And 2 Corinthians 4:16 also confirms this principle, saying the inward man is renewed "day by day." While once a day is the minimum, King David prayed three times a day (Psalm 55:17), as did Daniel (Daniel 6:13).
In considering what to say, Christ's model prayer in Matthew 6:9-13 gives us an outline of things we can pray about. For example, the first part of the prayer says, "Our Father in heaven, hallowed be Your name." To begin our prayers, we can address God as our Father, our Creator, who is in heaven. Hallowed means honored and holy. We can tell God that we are honoring Him and that it is our desire to continually honor Him. For each part of this model prayer, we can say these things in our own words and add our own thoughts to them.
Of course, praying for others is good. So is asking God to forgive us for our sins and mistakes. Sometimes we may come to God with a specific issue we want to pray about. At other times, we may wish to use this model prayer as an outline of things to pray about. On other occasions we may wish to turn to the Psalms and read some of them as a basis for our own prayers, interjecting our thoughts along with the words.
As for repetition, don't worry if you ask for some things on a regular basis. While God doesn't want us to use "vain repetitions as the heathen do" (Matthew 6:7), these are worthless repetitions because they are insincerely based on being seen of men rather than genuine, heartfelt concerns. If you are sincere about what you are asking, it isn't a vain repetition—it's a good repetition.
Don't get discouraged if you have a little difficulty in praying. Instead, persevere and keep trying. It will get easier the more often you do it. And be assured, God does appreciate our effort. In Revelation 8:3-4, it describes the prayers of the saints as mingled with incense (a sweet, pleasant smell) as they ascend up to God.