Like the others say, That bike's rubbish!
The big question is what are you looking to use the bike for?
If you're going off road into forest trails then you'll need to spend a minimum of £300(new) for a hardtail mountain bike (£500 for a decent hardtail).
Example of terrain below, when I was out with a group yesterday.
If you're just commuting on the road and riding along canal towpaths and other non challenging terrain then you should look at hybrid bikes instead of mountain bikes.
How much are you willing to spend?
Good brands are Specialized, GT, Giant, Mongoose, Claud Butler, Trek, Cannondale, etc.