What shode I upgrade first to run minecraft?

I want to play minecraft when I'm not working away from home but when I play it the know it is lagy where there are lots if stuff like jungles and that. Or even just trees.

This is ma pc willing I by new mother bored if must for a new cpu. It's has quet a lot of stuff on it would it work better as well If I clears it all back.
I run minecraft on all low settings and still lags. But when in a desert it's is playabil

eMachine E4231pecs

- Intel Celeron 420 1.6GHz Processor

- 1GB DDR2 Memory

- 80GB SATA II Hard Drive

- 56K* ITU v.92 ready Fax/Modem

- Intel PRO 10/100Mbps Ethernet LAN

- 15 in 1 Memory Card Reader

- Multi-Format Dual Layer DVD Re-Writer

- Intel integrated graphics with shared video memory

- High Definition 6-Channel (5.1) Audio

- 4 USB 2.0 ports, 1 Parallel, 2 PS/2, Audio-In & Out, Mic-In & Head Phone jack on front, Video ports

First thing should be your GPU. That's the most important thing for running any game. I would also put some more ram in there, at least 4gb, and if possible a new CPU, at least 2.4 GHz.

You need a dedicated gpu

usually it would be a pci 2.0 x16 (check your motherboard for copatability)
get a radeon 4xxx or better to run minecraft