I have observed that it is very tough to keep the mind free. I used to think too much and then get in to unnecessary trouble because of the same. I feel that the mind can be streamlined. Can you suggest the way?

First of all, it is important to understand what is mind. Mind is nothing but a collection of thoughts. Other than thoughts, there is nothing called mind.
Now, the next question is, how do these thoughts rise?
Thoughts rise because of Vasanas. Vasanas are nothing but latent impressions – impressions created deep inside us because we have been repeatedly doing certain activities for a long period of time. It is only when one is in deep sleep or when one is sedated that his mind does not function. During all other times, it is active. This is the nature of the mind.
Having thus said, it is also important that you don’t let your intellect watch the mind all the time and worry about your current state. This will create a complex in you, making you either guilty or proud and over-confident. So it is best not to police the mind.
Don’t mind what mind does.
The elephant cannot keep its trunk still. It keeps swaying it always. In order to arrest its movement, the mahout gives it a huge chain to hold on to. Similarly, in order for the mind to streamline its thoughts, we have it focus on the divine Name and Form of the Lord.
By chanting the Mahamantra either mechanically or whole-heartedly, the complex can be overcome and the thoughts can be streamlined. The Mahamantra has the power to do so.

I am not able to concentrate my mind when I sit for meditation or pooja. Some unnecessary thoughts come to my mind and disturb me. How do I get rid of them?

For a few days, chant the Mahamantra loudly in the mornings and evenings. That will make your mind quiet and bring ‘manolaya’ or calmness of your mind. That will help you get the required concentration to do mediation.

Are the soul and the mind the same?

No. Mind usually refers to the knowledge and is referred to as the ‘manas’. If you thus identify it, then it is completely different from the soul.

I have a very poor memory. Is there any special meditation or means to improve my memory?

Whenever you want to memorize something, follow the rules mentioned below. You will then be able to increase your memory:
Allot specific time to study and at any cause, don’t halt mid way.
Try to practice memorizing little by little. Don’t try to memorize all at once, especially when the subjects are similar in nature.
When you read, use the same textbook. Scan through the text in order.
Once you are done repeating your study enough, assume that everything is in your heart, and not in your brain.
When you are finished memorizing a chapter or subject and move on to the next one, repeat the sections already memorized by you.
Always have an optimistic thought that you have a tremendous memory
Imagine that you are storing all that you read in an imaginary storage rack in your heart and that you are storing your ideas in it in proper order.

I want to be totally free of sins and become joyful and cheerful, bring happiness to everyone in life. Knowing the questioning and doubting human mind, how can one ever get conviction in God, know Truth and become free?

Unless you experience the truth yourself, your doubts will not vanish. Great saints like Lord Buddha, Swami Vivekananda, Ramana Maharshi, J. Krishnamurtiji were terribly attacked by various puzzles, and their sincere quest led to their realization.
Before clarity, there is bound to be confusion. You should not stop with confusion. You must proceed in a divine path suitable to you and really verify for yourself, if the results experienced by these saints are really true.
Chanting the Maha Mantra incessantly with steadfast faith, patience and perseverance will help you achieve what you want.
It is heartening to note that you will stick on to the Japa and pray to the Lord to fill our hearts with love and noble thoughts. Its indeed the greatest prayer. We are very happy for you. Lord Krishna will bless you bountifully and will ever be with you in your spiritual quest.

Are the thoughts coming into the mind from outside or do they arise from our consciousness?

The stimulus (triggers) come from outside and they take the shape of thoughts inside us.

On some days, the moment I wake up, I have divine thoughts, and am immersed in the same bliss all through the day. On other days, I feel dry and empty – I feel deserted and am not able to think of the Lord. Why does this happen? What is the way out?

One some days, our body is very energetic to work, and on other days, its tired, isn’t it? The same is applicable for the mind too. Chanting certain specific ‘bheejakshara’ mantras as an upaasana without proper initiation is dangerous; performing breath-control practices on one’s own or taught by amateur masters is dangerous; similarly, analyzing the mind without bhakti also leads to a lot of confusion.
Believe in the grace of God as you take your life forward. Express your feelings to God with humility and love – that is real prayer. Continue to pour out your devotion to God and pray sincerely for his grace. With patience and perseverance in this spiritual practice, you will slowly realize that God is always with you.

After chanting for how long do evil thoughts cease in our mind?

It is not the count or time that matters. It is the intensity that matters.

Even by following the same routine, the condition of the mind is different on different days. What are the factors those make the mind disturbed on some days, joyful on some?

In Yoga and Vedanta Shastras, these concepts are explained in great detail. To put it in a nutshell, Vasanas and their provocations are relevant reasons for this. It takes consistent, persistent, long practice to attain one-pointedness of mind.

Adi Sankara says that intellect (koornda madhi) can be used for Realization. But Bhagavan Ramana says that the answer to Self-Realization cannot come from the mind, and that the thoughts (mind) need to be killed. One seems to be using the mind, while the other is aiming to kill it. What is the difference between the two?

With the sharper intellect grows the ability to discriminate between the real and the unreal. As long as you are not convinced about the what is real and what is not, your desire for Realization will not get serious. When the ‘viveka’ (sense of discrimination) of the mind improves, you will get closer to the Truth. Eventually, when realization dawns, even the mind will vanish.
Bhagavan Ramana gives a wonderful illustration in this regard – one of ‘Pinam chudum thadi’.
the interrer (‘vettiyaan’) in the cremation ground stands by the side of the burning corpse. Owing to the heat, the corpse rises up on its own accord every now and then. He uses a stick to beat down the dead body that rises. Finally, when the body does not rise up anymore, he throws the stick also into the fire. In the same way, a sharp mind aids in Self-enquiry to find the Truth and is itself eventually destroyed.