This is actually a very good thing to be asking about, as any kind of transition can be a difficult undertaking for even relatively experienced composers. (Personally, I think the reason why this is so is that 'classical'/'art'/'concert' music is a discipline that generally elevates the process of development across a temporal space, and the dramatic success of a piece within these parameters depends on how well linking together developmental ideas is handled, but this is a topic far beyond the scope of answering this question).
At the risk of stating the obvious: transitions range from the very subtle (two subsequent melodic ideas, in the same key and tempo in the exposition of a Mozart symphony) to the highly dramatic (the first movement of SIbelius' Fifth, which over 15 minutes masterfully changes from a slow and mysterious landscape to a breakneck tutti). Likewise, the methods of achieving them are varied, from gradual change to a sudden contrast. My feeling is that to the listener a successful transition is one that bridges two ideas by mixing elements of both. Study some minimalist piece for one method of doing this: the composer will often change one element at a time until after a while the music has moved away from the original idea. Note that this does not necessarily mean a complete change: some element such as tempo, underlying harmonic rhythm or pulse rhythm may remain the same. Of course a lot of what you can apply from the repertoire depends on what style you write in: the transition techniques of the baroque would not necessarily suit an impressionist work, nor those of high romanticism an electronic serial piece.
One thing I will say is that writing in a style which generally has a melody-and-accompaniment texture may make the task harder because of the expectations created in doing so. It becomes more difficult to use textural, motivic or timbral ideas, which can be a valuable resource, if you have set the music up as always having a tune on top. The other reason is that a tune presents the danger of being too 'stable' an entity to continue the piece from with any interest. Remember that the most destructive thing in maintaining the flow of piece is if it feels as if it has ended or come to a point where the material sounds settled enough to make it sound stalled. You need to keep some element of tension going to prevent the drama from being resolved and continue the piece. (Stravinsky's jibe that 'too many pieces finish too long after the end' is highly perceptive). One of my favourite things about Holst's Jupiter is that he doesn't bring the big tune back at the end. This is because it would upset the rest of the movement to have to transition into it again, as had he done so he would be backtracking, which would destroy the dramatic arc of the piece.
If I may, I would like to point you towards this workbook by a Canadian composition professor, Alan Belkin. Ideally you would start from the beginning, but Part 5 will be of particular interest as it sets several exercises in transitioning between ideas. There is also a section worth reading on his web guide to composition. https://www.webdepot...DF/FormWKBK.pdf. This also ties in to my final piece of advice: practice makes perfect. Not just as a beginning composer, but always. Personally, I always try to avoid using the same methods twice, instead I seek out new ways of solving a particular problem.