Which Strings Instrument to Learn?

The main strings orchestra instruments are the Violin, the Viola, the Cello and the Double Bass. 
Non-orchestral  popular string instrument is the guitar in all its forms - classical guitar, folk guitar, Spanish guitar, electric guitar and electric bass guitar.

• Violin
can be started at a very early age and it doesn't cost as much as most other instruments. Later on it offers excellent opportunities to play with other people as orchestras need an awful lot of violinists. It's of little use in wind bands, brass bands, jazz bands or rock groups (although on the commercial music scene there has been an upsurge of interest in the Violin lately, often using Electric Violins. These aren't any easier to play and are not suitable instruments for beginners, though)

• Viola
can be started at a very early age and it doesn't cost as much as most other instruments. Later on it offers really excellent opportunities to play with other people as orchestras are always short of violists. It's of little use in wind bands, brass bands, jazz bands or rock groups

• Cello
can be started at a very early age and, while expensive compared to the Violin or Viola, it costs no more than many woodwind or brass instruments. Later on it offers excellent opportunities to play with other people as orchestras need an awful lot of cellists. It's of little use in wind bands, brass bands, jazz bands or rock groups

•Double Bass
can be started at an early age if you can get a small enough instrument, but is usually started at age 12-or-so or even later. It's a very suitable instrument for those who play a non-orchestral instrument (say, the piano) and want to adopt a "second instrument". It is fairly expensive. Later on it offers excellent opportunities to play with other people as orchestras are always short of bassists. It can be used in wind bands, jazz bands and sometimes in rock groups. It is exceptionally difficult to transport and will make heavy demands on the "parental taxi service"

• Guitars
can be cheap to buy.
Classical guitar is difficult, Folk Guitar is not so hard. The Electric Guitar can be used in jazz or rock groups and sometimes in wind bands. The Classical Guitar does not offer good opportunities to play in groups - it's too quiet.