the diamond back probably has a cup and loose caged bearing bottom bracket. Very easy to fix. You can get new cups if required and bearing's ( in a cage ) for about £10 online or at a shop. Alternatively new bearings alone( in a cage )are a couple of quid from halfords. Just lossen bolts and hammer the ring with a screwdedriver to take apart. New cups ( if required ie split or worn) can be hammer out / in with a socket set extension / hammer and wooden drift. Halfords also do cheap taya chains for BMX's that'll fit yours. If the tyres have had it you can get cheap ok ones from JJB sports ( i got kenda's street tyres for my skip rebuild) or online should you have a local branch. Personally I'd keep the front brake on unless the back is really good (which i doubt)
Not topo sure how to fix the rear hub. May need a wheel?? From a tip?? As it a dimanond back you may be able to get new cones / freewheel etc.
If the giro ( brake system ) does not work I'd just take it off and a run normal cable. The weight of the bike will be a good thing to as you will get better at putting the power down on mtb.
Bear in mind that you ok beginer bmx's off ebay cheaply so I wouldn't spend much on it
Cheap Pub / Shopping bmx's are ace 9 unless you have to cycle miles 😀