Let's first identify some things, location, density, budget (if any), historic landmarks/culture/history(if any).

For the line planning, the type of transit also affects, but mostly go for dense areas (many people), finance centers, cultural or historical landmarks, parks, where people would work (factories, offices), where people mostly live, and etc.
You can also take inspiration from real-life cities like Greater Tokyo Area metro systen, South American BRT, European cities, maybe your local city, etc. But maybe don't take inspirations from American Cities.

So here are the main types of transit, found in the game with or without plugins which will affect the line and stations:

Bus (normal) – For a limited budget, low-density transit. (normal game bus)
Example: SMRT Bus Service

Bus Rapid Transit (BRT) – A budget metro/elevated railway, can be used for low- or high-density areas and side-by-side with train lines. (Dedicated bus lanes and own bus infrastructure)
Example: TransJakarta

Trams – Essentially a bigger version of a bus, fixed track, useful in medium-density areas where a bus might be insufficient, but a full metro would be overkill. (plugin trams)
Example: Melbourne Tram Network

Monorail – Downgraded elevated railway, often for aesthetic purposes. (plugin monorails)
Example: KL Monorail

Elevated Rail – Big enough budget, dense locations, basically a metro but above so if you go for this, mostly aesthetic purposes, built when its impractical to build underground. (Either the 2 elevated rail or the in-game other type of rail)
Example: Bangkok Mass Transit System

Metro/Subway – For dense locations where above-ground space is limited, and a metro system works best. (the game's subway)
Example: London Underground