The pick you use has a huge impact on the way the guitar sounds. Experienced flatpickers recommend using a thicker pick. This is because a thick pick allows your guitar to resonate more. It plays louder than a thin pick and it brings out the lower notes. Make sure you're hitting the strings at a bit of an angle, and not "slapping" them straight on.
When playing lead guitar, there are a lot of notes to hit. This requires lots of strength in your left hand (or right hand if you're left-handed). Resist the temptation to be lazy. If you don't press the strings all the way down, the riffs will come out muffled. If you're having trouble with this, you may want to start with light-gauge strings. As soon as you get the hang of it, switch to medium-gauge strings. They have a fuller tone.
An Internet search will bring up hundreds of free flatpicking lessons and tablature. Use videos to get a feel for what your guitar playing should look and sound like. Find songs by The Carter Family, Doc Watson and Tony Rice. Learn to play classic licks and songs. These will help you build your technique and serve as inspiration for applying flatpicking technique to original compositions.