It really depends on your style. You should check different options. But you must be warned - as a beginner you probbably won't be earning money! Noone is playing poker to give their money to you. You will need to spend some money before you get the experience and become a winning player.
Cash games - they teach you patience and responsibility because you usually play with deep stack (which doesn't mean large money, a deep stack can be 2$). With cash game you can usually just wait for premium
hands, which in general is easier poker. Much slower paced than other options. You can also learn to lay down strong cards (like two pairs) which is almost impossible in tournaments where you just can't afford to do it and have to gamble.
Single table tournaments - they teach you some basics of tournament play and don't require large bankroll. For example for 0,25$ SNG you probbably need around 5$ of bankroll (20 buy-ins). Early stages look like cash but then you will need to change gears into much more agressive style and do some gamble. They are easy to actually check if you are a winning player - after you've played 100 or so it should be obvious. It is also important to play those to prepare for final table on normal tournament and to learn final heads-up game.
Normal tournaments - they are the ultimate poker activity. Require certain skills. You will probably need a lot of bankroll to play those (50 buy-ins or more), because they require a lot of luck. You will usually bust early but once in a time you can get to the final table and payouts are then really nice. Due to large variance it's really hard to tell if you are a winning player because a winning player can easly not get any money 20 times in a row, while a bad player can win a lot in his first play.
Freerolls - this is an interesting option if you don't want to put your own money to play. Early stages seem like play money (people just don't care because they signed in for free) so it's not entertaining neither you will learn anything, but as you progress to the later stages near the bubble people start to play better, like in a casual tournament. Unfortunately the prize poll is really low - for example 100$ for few thousand people - your expected EV is dramatically low (but still positive no matter how bad you play!). Many people consider them a waste of time.
Cardschat Freerolls - Much better option. Better players and lower player poll gives generally much better competition and expected payouts are waaay better. Especially if you can earn platinium.
Spin&Go - rather losing option but if you want to train heads-up (which is essential in winning a big tournamet) or you only have few minutes of spare time, you can try those. But they are rather for fun than actually earning anything.