Hi, A few basic tips

1. Rivers flow from high places into the sea. Source your rivers at a lake, or glacier, or in the mountains or hills and have them flow into the the ocean, branching as necessary. Remember that they tend to flow from high to low areas (gravity being what pushes them down). Since sea level is the lowest area it will flow into the sea.

2. Depending on the time period, most of important cities are either going to be coastal or based on major rivers especially in pre-modern settings (where irrigation systems are not as complicated)

3. Separate the map into the following paradigms: Wet and Dry areas and Low and High areas. This will make it easier to create transition zones between the different paradigms.

4. Alternatively, a mountain range can be used for a sharper transition, as it can indicate that the two different biomes are, in fact part, of separate plates.

5. Creating landmasses is difficult but there's a lot of programs and internet sites that can generate a basic landmass for you. I found it helpful to use a generator for ideas then add distinct features as necessary. Although a lot of hubub is made about realism both in this post and in this thread, cool features are what make a map striking and memorable in your novel/game/rpg campaign. If you've read The Stormlight Archives think of the Shattered Planes. Is it a realistic geological feature? No. Does it look awesome on a map? Hell yeah.

6. Additionally, story convention is always, always, always more important than realism in a fantasy map. Realism is a tool, not a goal.

7. You can use your map to tell the story of the area. In fantasy, magic can twist and shape the landscape in odd ways. Have that play out on the map. Giant craters, sweeping mountains that form a wall, what have you. The magical nature of fantasy lets you have some fun with the geography.

8. Alternatively, geography plays a role in shaping the major powers of the continent. The premier power in the world should not be an overland empire with no warm water port, with a capital (or at least a de facto one, de jure might be a different story) in the middle of butt-all nowhere. Large cities tend to be the ones with warm water ports. For a real life example of this, look at Constantinople. It was one of the largest cities in the world during the medieval era (typical fantasy) and it's no accident. Look at Istanbul on a map. It's a dream location, warm water port and surrounded by fertile farmland that can sustain a city of that size. Of course, this all gets more complicated when magic gets involved, but large cities still tend to form around these regions. Universal access to food hasn't made New York or Tokyo shrink so keep that in mind.

9. Major powers are the ones who have high population and can move their armies. That's why the "sea power" is so stereotypical it's almost a cliche. The ability to move your armies is a big part of military dominance and is easily linked with expansion. It's how the vikings invaded Sicily and Russia and why the English conquered such a large part of the world. Geography plays a role and in big a nation can be.

So there are a few basic tips to get you started. Feel free to message me again with any questions and I look forward to seeing your work.