Can you use an old lense on a Nikon dslr?

I have a old Nikon Em slr and wanted to know if I could put that lense on a Nikon d3100 I'm getting in to dslr's and don't want to spend to much. Also I have a tripod with a tripod mount that screws on to the bottom and wanted to k ow if it would fit onto a dslr.

Yes. And probably yes.

Nikon users have one advantage over Canon users. Nikon's mount has not changed, so with only a few exceptions any lens that has been ai'ed can be used on any Nikon bayonet mount. This does not apply to Nikon's APS format SLR lenses.

Some auto functions will not work. You can Google Nikon lens compatibility and see what will work with which lens.

Tripod mounts are pretty standard, so it should fit.