Generally speaking, the first upgrades for a budget bike are wheels and tires
.First upgrade your tires. A light weight quality tire and lightweight inner tubes will reduce the weight significantly at the rim and will make a significant difference even on budget wheels.
Four tires worth considering are:-
Continental 4000s (209.5g). Continental Ultragator (217g). Schwalbe Durano s (222g). finally my favourite, Schwalbe Ultremo R1 (187g) (some offers around on these at the moment)
As for inner tubes, Continental Race 28 Supersonic (50g) are very good.
As far as wheel upgrades go, good possibilities are:- Shimano R500 (2000g), Fulcram 5(1809g) and Shimano Ultegra 6700 (1652g). There are others but the prices begin to climb.
I would go for the tires and tubes first and see how you get on.