1. Place the strawberries, sugar and lemon juice in a microwave-safe bowl (3 liter or 3 quart).

2. Cover with plastic wrap and poke a hole in the center to release the steam. Cook on HIGH (1200W) for 2 minutes.

3. When sugar melts and starts to boil, unwrap the cover.

4. Cook on HIGH (1200W) 2 more minutes to finish.

5. Take out and let it cool. The jam will thicken as it cools.

For how long you should cook with your microwave oven, see Cooking Time vs Wattage Conversion.
For more tips of using Microwave Oven, see Sachiko's Guideline to Microwave Cooking.