Always sand with the grain never across it. I start with 100 gr it if it's really beat up. If it's in decent shape I start with 180 gr then 220 gr and finish with 320 gr. I pour alcohol on it and dry with a heat gun as that raises lil slivers and re sand. Careful not to round any edges. I tape off any checkering while I'm sanding.
With tru oil the more you add the deeper it gets and the shinier it gets. 0000 steel wool will dull the finish to a mat finish if you wish
Larry potterfield from midway USA has some great videos detailing how to do just about everything.