Science & Industry

Salvaging can be far more then a mini profession.

Build up a noctis with T2 salvagers and lots of cargo space.

Use hardwirings that improve salvaging.

Look for mission runners who will leave you bookmarks full of wrecks.

Split the loot/salvage with them and build a a reliable client base.
Join a mission corp that will provide some extra work if needed.

Stick to one major mission hub and salvage so fast that you can work for multiple people at once.

You'll can see a good 25mil an hour in high sec.

If you want to expand from that point start building rigs and working on trade/reprocessing skills.

For steady and decent isk income:

Mining - use frig, then cruiser, then ORE ships if you want to go all the way. Also train refining skills so you can refine your ore into minerals which sells better than the chunks of ore.

Missions - very easy and the combat skills you train will give direct impact on your efficiency. Simple rule of thumb is Level 1 missions: Frigate, Level 2: Cruiser, Level 3: Battlecruiser, Level 4: Battleship. Optionally you can do the missions for Distribution agents which mostly involves transporting items from one station to another (no combat) or Mining agents (which send you out to mine X amount of a special type of mission ore that can only be used to finish the missions).

Planetary Interaction - Not huge amounts of isk, but as others said, once you have set up a small colony to extract resources it will just keep going, and going... so it's a steady, low-maintenance way to make isk. All you really have to do is just go pick up the goods once a week, or once every two weeks (depending on the cycles you set) and go sell it.

Building and trading takes longer, and also requires cash to get started. To do building you need to first invest into a blueprint that you can use to build. You then need to either mine the minerals required, or buy them off the market. Same thing with trading... simple rule of economy, it takes money to make money.

Researching / Invention takes even longer. Partly because the skills can be slow to train, and mostly because the Research slots in regular stations are usually full, and many have queues that are 20-30 days long (which means it takes 20-30 days before your blueprint is even started on). If you join a corp that has a POS with free R&D slots then this is a viable option, but if you are still in the starting NPC corp, then this is no way to make good isk early on.

Exploration is a be lucky or be screwed kind of way to make isk. If you want to do exploration you need to train the main Astrometric skill to 4 and the Astrometric Pinpointing and Rangefinding skills to at least 3 (Astrometric Acquisition you can leave at level 2, it's mostly useless). Then you need hacking, salvaging, archeology and combat skills (or an alt with a combat ship). Exploration is not about a person being specialized into just hacking. You train all the skills so you can enter and grab the loot from all the sites you find. Why limit yourself to only Radar (hack) sites if you can also grab Magnetometric and Unknown (combat) sites ?

But exploration is all luck. Either you find something worth alot, or you spend days and find nothing.

All in all, the most common way for new players to make ISK is by running missions or mining ore.