Hi Kate
With the budget you have you'll get a pretty decent hardtail mountain bike (front suspension only). If you'll also be using the bike on country roads maybe look for front forks that lock out, it's easier to ride on tarmac if the front of the bike isn't bobbing about.
Where in the country do you live? Somebody local to you on here might be able to suggest a good bike shop near you. Places like Evans are quite good for being able to compare a lot of bikes, and get an idea of what you get for your money. Any of the big brands will be a good bet for the first bikes to be looking at Specialized, Trek, Kona, Giant, Marin among others. Be sure to look at the womens specific models too as these might fit you better. A good shop will be able to advise you on fit.
Hope that helps a bit!