how to smooth and level bedroom floorboards

[fearofdiy]

Hi,

So my daughter is away for a long weekend and thought i would sort out her room as he just bought a house and it is in a terrible state. I just pulled up the carpet to find the floor boards underneath are in pretty bad shape. MY question is can what do i need to do to make all this smooth? A sheet sander or a belt sander? the uneven floorboards seem like a problem however, a bed will cover that and I had not noticed before I pulled up the carpet just now.

http://imgur.com/bEujVMK

The plan is to paint the walls first, and then sort the floor but i am a bit of DIY noob so should I go for a belt sander or a sheet sander? the job does not need to be perfect just habitable for a 8 year old girl

any help appreciated.

thanks