First of all,I think one must have financial stability.
Anyone who wants to invest, whether he is financial stable or not can be an investor. If he can invest small amount that can be his own start to learn more about it and gain financial stability.Have to take risk of certain amount that won't fall any impact in financial stability.
Like invest only what they afford to lose.What others thing one must know?
Before anything else like the amount they are investing or how much they can invest. They should be financial literate and have basic knowledge about proper investing. Literacy in investing is needed because someone who has big money but isn't literate with it can also lose money.