I only want to add something to what has been explained beautifully by Venkiteswaran.
Along with choosing a good and strong company one should also choose a good time and opportunity to invest in it.
Coming back to the question,
1. it is of utmost importance to see whether the promoters have a good reputation for giving back to the investors. I mean dividends, bonus, splits, rights issues etc.
2. If a company has a monopolistic hold of market share in the field in which it deals, it may be regarded as a strong buy if you invest in it.
3. Along with steady profits as already explained by Venkiteswaran, if company's sales figures have a steady up-trend then the company may be regarded as a strong company.
4. If promoters hold more than 45 to 50% shares or are attempting to increase their share of holding one can be sure it will prove to be a good buy.
5. If a stock is selling at a price below or equal to its book value it may be a good opportunity to buy it given other factors as explained above are also at reasonable level.