i am now struggling on the transition part now, can you suggest a chord progression that can move from 1 key to another smoothly for me?

the original key is in D minor so the transition should starts in A minor

I am unclear as to what you are trying to do. Are you trying to compose a sonata exposition in the style of Mozart?

Have you look into Mozart's sonata expositions in minor keys? In the A-minor Piano Sonata, the second theme group is in C major. In the C-minor Piano Sonata, the second theme group is in Eb major.

Perhaps you should try to write the second theme group in F major if you are starting the exposition in D minor. Also, why should the transition start in the new key? As the term implies, it should MODULATE from the original key to the new key, not changing keys abruptly. It is best to end the transition on the V chord of the new key.

Little trick for modulating, it usually works. before the chord V of the new key as mentioned above try to find a chord that common to both the key you have come from and the key you are going to. So for instance if you were to go into A minor (although I agree it would probably make more sense to go into F major (the relative major)) then you would say have chord ib in D minor then chord V in D minor then maybe chord i of D minor which is chord iv in A minor (because it is DFA for both) and then go to the dominant E major and then to A minor. There are loads of ways to modulate including by sequence and by held note but this one works quite well I think.