Adding a mirror modifier.
1. Press Num Pad 1 to go to front view. Select the left half of the model using B. Press X or Del > Vertices.
2. Switch to the Editing Tab using F9. Scroll to the right and click on Add Modifier > Mirror.
3. Merge Limit defines the distance that keeps vertices from joining. 0.0001 is good. Activate Do Clipping.
4. Optional: If your project is a character, make the Armature modifier real by clicking Make Real next to Armature parent deform. Now click the down arrow of the Armature modifier so goes downstairs and the Mirror modifier becomes first. Never apply the Armature Modifier, unless you want to create a statue!
Applying a mirror modifier.
1. Before exporting, the mirror modifier needs to be applied. Thus click Apply.