I want to change a 2 wire outlet to a grounded 3 wire outlet

If there is a ground to the box, all is well. If not, that's another discussion.

If it's wired with 14 AWG and fused at 15 A then use an outlet rated for 15A.
If it's wired with 12 AWG and fused at 20 A then use an outlet rated for 20A.

The wires need to be transferred to the new outlet. Black to the brass screws and whites to the plated screws.

The ground (green screw) on the outlet needs to be connected to ground. Both the metal box and the green screw need to be connected. This is normally referred to as "pigtailed". The hardware store has short lengths of this wire available for purchase.

It is possible you have a ground to connect the outlet ground. Most homes are wired up with NM (Romex), and in the past much of it didn't have a ground wire. If you have NM and can't find a ground wire, the only way you can have a grounded outlet is to pull a new cable. A few homes are wired up using BX or metallic conduit. Properly installed, they provide the ground you need.

Your last option to gain protection is to install a GFCI outlet and mark it no equipment ground. That does not give you the ground some electronic equipment or surge protectors need, but does protect you from short circuits.