There is a huge difference between a nervous tremor and vibrato. I agree with John Hardly - not a single good well known singer doesn't use it. But too much is horrible. It is more a question of tone and control and can be used very effictively to sweeten the end of a long note. If used on every note (opera spings to mind) it gets boring. Listen to a good fiddler playing an air - he will play many notes straight and open up on a few with vibrato - very effective. I learnt by immitating my favourite singers - of course it feels strange at first but if you work at it it will become regular and sweet. It is definitely a good idea to get lessons from someone who know what they're talking about but take care not to have your voice forced into a style different to the one you like - listen to an opera singer singing a folk song if you want to know what I mean.