I am making RPG in OpenGl and I need to make some portals. How should I render it if I want to see through the portal on the other side?
You have two scenes rendered from two camera perspectives: one in local space and one in portal space. You need to superimpose part of the latter onto the former. You need a working knowledge of G-buffers (deferred shading) to achieve this, so I suggest you begin your research there.
First shader pass: Construct a G-buffer for your local scene. In your specific case, you'll have a colour buffer dedicated to masking out the silhouette of the portal from a local perspective; it will contain a colour (say white) denoting where your portal surface is, with everything else in local space being rendered black. This mask is used so that you know which pixels of your rendered local scene should be considered as a window into "portal space".
Second shader pass: Render your portal space scene with same camera orientation, but camera position shifted by the same vector that describes the camera's offset from the portal in local space.
Third/final shader pass: Use mask to cut out the section of the portal scene that you need to super-impose over the portal surface in the local scene, and super-impose in the fragment shader.
DISCLAIMER This is all easier said than done - getting into multi-pass rendering and g-buffers is a considerable amount of work if you've never done it before - expect several long days to several weeks depending on your level of skill, available time and general luck during implementation. Also, understand that deferred shading has a high cost and may be unsuitable for certain mobiles.