That's the million dollar question Chief. There's a number of things you can do and if you just started guitar don't expect to be shredding like Slash or anything just yet. It takes time and patience. A metronome will be your best friend with devoping speed so get one and practice with it but remember. Start slow even after you've accomplished something you can handle. When you start practicing start at a speed you know you can take then work up. If you're having a hard time keeping up with something then go back down a bit and familiarize yourself with something else for a bit. Just remember, you can't play fast if you can't play slow. If you feel like you're straining yourself then slow down and relax. Tension is also bad and causes you to slow down. If you tense up then you've got resisting force to work against and you can not only decrease your speed but damage your tendons and muscles. Just take it easy and as they always say Practice, Practice, Practice.