This is gonna be a little guide so minecraft doesn't lag when your playing.
The first thing you might wanna do is open up task manager (Ctrl+Alt+Del), head over the the processes tab, and start closing unused processes, make sure to read the description so your not ending stuff thats important, if you don't know what it is, don't end it... better safe then sorry.
If that doesn't work you may wont to hit the Escape key while playing minecraft, open the options box, and select the video settings.. now here is the settings you may want...
Limit Framerate: Off
View Bobbing: Off (unless you know what that is...)
Render distance: Normal-Tiny
3D: Doesn't matter
Smooth Lighting: On, it only takes off like 1-2 fps
All of this info should reduce the lag while playing minecraft, if you still are lagging, post on this topic and I'll try to help you out.