Entry by Qualifying Standards
In decathlon, your ability compared to the average athlete is irrelevant; what you need is to meet the fixed entry standard within a certain time period prior to the Games, for yourself.
The entry standard for the 2016 Olympic Games decathlon event was
a score of 8100 or greater,
obtained at an IAAF-approved event
between 1 January 2015 and 11 July 2016
This gave about 18 months of events, and qualifying ended four weeks prior to the opening ceremony.
A similar standard and qualifying period can be expected for the 2020 Olympic Games; you could need a score notably above 8000 at an IAAF event during 2019 or 2020 prior to the Games' beginning.
Entry by Nomination
If you do not meet this standard, there may be one "saving grace" method of entry.
For the 2016 Olympic Games, contrary to prior Games, a National Olympic Committee which did not have any athletes meet the entry standards in one of a list of specific events, could nominate a single athlete (in each sex, as and where appropriate) for those specific events not qualified to. This guaranteed every NOC could have at least one athlete in every event of the athletics components of the games, if they so desired.
It is not known whether this alternative method of entry will be in place for Tokyo 2020 Olympic Games. You should aim to learn and to meet the full qualifying standard regardless of whether others in your country can or cannot also meet it.