I have to install a 3 prong receptable in place of an old two prong on a 20 amp circuit. I realize that I should replace the old two- wire with no ground with #12 two- wire and ground wire. The new wire will have to be connected to old two- wires in a junction box. Where do I connect the ground wire in the junction box?

Don't quite understand your post.

You either have a ground wire or you don't. If you don't it is permissible to install a GFCI marked "GFCI protected. Ungrounded".

You can install 15 or 20 A receptacles on a 20 A circuit.

The breaker chosen is dependent on wire size. 15 or 20 Amp for 12 awg, 15 Amp for 14 AWG

Ground clips and ground screws are available to use. Pigtailing connections are preferred.