How do I choose the right AWS EC2 instance type?

I read the AWS blog Choosing the Right EC2 Instance Type for Your Application, but I am still not sure how to choose the right one in terms of the cost-performance.

I have several WordPress ruing on the AWS EC2 t2.micro instance. Recently, I often encounter the issue Out of memory: Kill process ** (apache2). The AWS free tier is about to expire. I was wondering which AWS EC2 instance type I should purchase? What indicators are used to make a right decision and how?

I have a main wordpress blog, according to Google Analystic,

Here is a part of CloudWatch (last 24 hours).

Runing top on my Ubuntu server sorting by the memory usage.

$ top
top - 21:53:01 up 2 days, 22:20,  1 user,  load average: 0.01, 0.03, 0.05
Tasks: 103 total,   1 ruing, 102 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.4 us,  0.0 sy,  0.0 ni, 99.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   1016292 total,   644428 used,   371864 free,    64996 buffers
KiB Swap:  1048572 total,    84308 used,   964264 free.   230348 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND                                                           
10044 mysql     20   0  901108  86808   3336 S  0.0  8.5  18:05.58 mysqld                                                            
26417 www-data  20   0  446684  84072  35700 S  0.0  8.3   0:04.87 apache2                                                           
26420 www-data  20   0  454500  82936  26752 S  0.0  8.2   0:09.43 apache2                                                           
26419 www-data  20   0  440836  68820  26308 S  0.0  6.8   0:10.74 apache2                                                           
26468 www-data  20   0  431884  68332  34812 S  0.0  6.7   0:06.02 apache2                                                           
26484 root      20   0  105628   4216   3232 S  0.0  0.4   0:00.00 sshd                                                              
 1974 root      20   0  396220   3964   2260 S  0.0  0.4   0:12.14 apache2                                                           
26568 ubuntu    20   0   25924   3792   1708 S  0.0  0.4   0:00.04 bash                                                              
26567 ubuntu    20   0  105628   1892    904 S  0.0  0.2   0:00.00 sshd                                                              
26598 ubuntu    20   0   28084   1552   1116 R  0.0  0.2   0:00.01 top