A tonal chart will help you pronounce the four tones. When just starting Chinese, it's better to use somewhat rigid but correct tones. After you are more familiar with the way Native Chinese speakers slightly alter the tones but not change any meaning, it will be okay to be a bit relaxed with the tones. Right now, we want to speak slowly and correctly.
Don't be intimidated by how fast native speakers talk to each other. Their ears have been trained to pick up any sound close to Chinese words and interpret them correctly in context. This takes years of listening to other Chinese speakers, watching Chinese movies, listening to Chinese news, radio, and songs (especially songs, where the tones are completely messed up, they can stil pick out the meaning using contextual clues. I am a native speaker and I can't even tell what the Chinese singers are saying sometimes). This is why most Chinese people can understand chinese spoken with accent. They might need to think a little about your words with the messed-up tones, but it should be easy for them to interpret and find the approximate-meaning, the same way you can tell what a Japanese woman is saying when she speaks with heavy accent, but only with a bit of effort to"figuring it out".
Learning the tones correctly is a real wow factor. Chinese people just can't believe how an American can say things exactly the way they should be said. They will even question if you were an American :)