You MUST know how much is in your account. If you do, and you risk a fixed amount every trade, you can get your risk percentage;
Account balance, say $100.
Fixed risk amount, say $3
Percentage you are prepared to risk =3% (100/3)
You MUST know what you are trading. Are you trading Aud/Usd, Usd/Gbp, etc?
Once you have your account balance, risk percentage, and pair you are trading, you can use the free calculator here on BabyPips to get your position size