First you need 61 normal logs to get it to 15 which is 2411 XP at level(Runescape Gold) 1. There have 61 normal logs cost 1830 gp or you could just cut them yourself. You will get 13,363 XP when you are level 15 with 183 oak logs or 274 normal logs.
When you are at level 30 in Runescape. You will need to got 535 willow logs, 803 oak logs or 1204 normal logs which can give you 61,512 XP from firemaking. 1573 maple logs, 2359 willow logs, 3538 oak logs or 5306 normal logs will be need to get when you at level 45. From these you will have 273,742 XP(runescape gold).
With your level(rs gold) up, you will need more. 4626 yew logs, 6939 maple logs, 10,408 willow logs, 15,612 oak logs or 23,417 normal logs for the level 60. And the exp will be 1,210,421 at level 60. You need 38,927 magic logs, 58,391 yew logs, 87,586 maple logs, 131,378 willow logs, 197,067 oak logs or 295,601 normal logs when you at level 75 and the exp is 13,034,431 XP.You can buy runescape.