Welcome to my Beginner's Farming Guide for OSRS.
There's 3 methods that I'll show, Questing, Hosidius Favour and Bagged Plants.
Questing is a great way to skip the early farming levels.
Questing for XP
If you complete the Fairytale Part 1 quest, you get from level 1 to 17 farming, and it has low requirements. Level 17 farming is enough to start Tree Farming, but if you're into questing, You can complete these quests to reach level 38 farming from level 1. You should at least do The Garden of Death, an easy quest which gives 10 thousand farming XP.
Hosidius Favour
While getting Hosidius Favour, you can train your early farming levels very quickly
You firstly need to plough the field to reach 5% favour.
Then you combine saltpetre and compost to make fertiliser all the way to 100%, which gives farming experience. You need 950 fertiliser to reach 100% favour, and doing that will get you from level 1 to 13 farming.
To get to level 15 farming for Oak Trees, you can use a Garden Pie to boost while you're planting the patch. These are available on the Grand Exchange and are useful at all farming levels to plant things you normally couldn’t.
Bagged Plants
Low level farmers can also train with bagged plants.
To do this, you need a house, filled watering cans and bagged plant ones. Then in building mode, you repeatedly plant and remove the regular plant in your portal room.
You need 78 bagged plants to get from level 1 to 15 farming and these can be purchased from the Grand Exchange,
but I'd suggest buying them from the gardener in Falador or the Farming Guild for a lower cost.
For an in depth guide on this training method, I made a video a few years back which is linked here.