To start with, find out how much you can allocate for long term investment. Then think of how much you can allocate on a regular basis because you cannot just buy once and leave it. You should have capital to buy later as well when there is a correction and good stocks are available at attractive rates.
Next is identifying a sector that is likely to grow. In Indian context, finance sector is great and so is consumption. For long term investment, these sectors are really good. Over a period of time, you can get great returns. However, the most important thing in this is the selection of stocks. Each sector has stocks that you can regret buying later.
On the other hand, each sector has stocks that can give stellar returns. Think of a sector like technology and how these stocks have struggled. And yet TCS and Infy can still give great results. Banking sector is struggling too due to NPAs and other issues. Yet, HDFC Bank and Kotak Bank have given great returns.
You need to make sure that you select the right stocks and stay with them for a longer horizon.
Once you select right stocks and invest long term, the investment is going to eventually pay off!