The past simple tense describes an action that took place sometime in the past, while the present perfect tense describes an action that started in the past and is still ongoing or the effect of an action is still present.
1) Finally, I started (past simple tense) reading English novels with the help of my dictionary.
(You started reading your novel sometime in the past. For example, you started reading at 10 p.m. last night.)
2) Finally, I have started (present perfect tense) reading English novels with the help of my dictionary.
(You started reading your novel in the past, and will continue to do so. For example, you started reading at 10 p.m. last night, and you are reading the second chapter now.)
Just as a side discussion, if your sentence is changed slightly to: "Finally, I _________ reading English novels after dinner last night with the help of my dictionary.", it would be more appropriate to use the past simple tense since a starting time is included.
A few more examples:
1.Reported speech: Mrs Tan asked Alan whether he had visited his grandparents the previous Sunday.
2.Direct speech: Mrs Tan asked Alan, "DID YOU VISIT your parents last Sunday?" The use of past simple tense is more appropriate as a past time is provided.
3. Reported speech: Tom asked me if I have started work since the previous month.
4. Direct speech: Tom asked, "HAVE you STARTED work since last month?"
The use of present perfect tense is more appropriate as "since" describes a period between (the time mentioned) and the time under consideration, typically the present.
I hope this helps.