Hello Satbir,
I see you've already received a number of excellent answers to your question so I won't repeat what's already been said.
The main problem most new investors face is how to handle contradictions expounded by the various experts.
Just as there are many types of people in this world, there are many types of investors who follow many different types of investment strategies.
And many of these strategies contradict each other. It's funny because everyone's ultimate goal is to end up with more money at some point in the future, but the path each type of person takes to reach that goal is vastly different.
So, if you're new to investing you need to determine which path you want to take. Is it a short-term trading strategy, do you want to day trade, are you a buy and hold investor or something in between? (Keep in mind that you can mix and match strategies if you like, for example your retirement savings might be managed with a long-term strategy while a portion of your non-retirement funds might be day-traded.)
As soon as you decide on your strategy, things become clearer.
Of course some things are relevant regardless of the strategy you choose to implement.
First and foremost is risk management. Most new investors focus solely on returns. That's a big mistake. Whether you're day trading or investing for the long-term, you need to focus first on risk and then on returns.
AIM provides one type of risk management and as such you need to feed it stocks that can be used with that risk strategy. Mainly that means selecting undervalued, fundamentally solid stocks that are volatile and then diversifying and allocating appropriately.
If you have limited funds, then sector ETFs are great because they generally have low expenses and provide instant diversification.
Note that I said sector ETFs rather than just ETFs that mimic a broad index.
The reason is to attempt to capture volatility. If you simply invest in a broad index, much of the volatility of its components will be cancelled out. By splitting into sectors, you should get diversification plus increased volatility.
If you have more funds (e.g. $30,000 +) and some experience behind you, then individual stocks are the way to go (that's my opinion only, you have to decide for yourself if you're comfortable with individual stocks).
By selecting undervalued fundamentally sound stocks you limit your risk when the price drops. Good stocks can drop as we've all seen, but because of their earnings over a relatively long time frame, they tend to hit a bottom and then rebound.
Fundamentally shaky stocks (and stocks that are overvalued, whether fundamentally sound or not), tend to drop and not recover or are either delisted or go bankrupt (these are not good candidates for AIM, although they might be excellent candidates for other strategies).
So if you've done your homework and selected the appropriate AIM stocks, you should not have to worry about purchasing when the price drops, because you know that over time it should come back up (if that's the case, then you might as well pick up some bargains along the way).
Compare this with a short-term strategy. If you're in for a day or a week or 2 weeks, then if the price starts to come down, you will most likely want to cut your losses and sell. That's because your time horizon is so short that you can't be sure the stock will rebound in the next day or week or 2 weeks.
If you're following this type of strategy, then you would definitely not want to average down by purchasing more. You'd simply take your loss and find the next short-term darling.
To summarize, if you've decided to use AIM for all or part of your investments, then the following steps should help....
1) Have a longer term time frame (4+ years) in mind.
2) Select only undervalued, quality stocks that are fundamentally sound. Or select sector ETFs.
3) Diversify according to a proven strategy (generally you should look to hold stocks/ETFs with low correlations).
4) Allocate according to a proven strategy (there are many ways to allocate).
5) Manage your equity positions with AIM (although there are a number of AIM variations, stick with a By the Book strategy to start with until you build up some experience).
6) Periodically check to ensure your selected stocks haven't changed for the worse fundamentally or become ridiculously overvalued. If so, find a new stock to replace it (if you're using sector ETFs, you don't have to worry about step 6).
And that's about it in a nutshell.
The other thing you might want to check out is Tom's Idiot Wave. It gives you an indication of how much equity (as a percentage of your initial investment amount) you might want to allocate to equities.