Basic Loop
#!/bin/bash
for i in 1 2 3 4 5
do
   echo "Number $i"
done

Output: 
1
2
3
4
5

Using seq
#!/bin/bash
for i in $(seq 1 2 20)
do
   echo "Number $i"
done

Output: 
1
3
5
7
9
11
13
15
17
19

Using the C-style loop: 
#!/bin/bash
for (( c=1; c<=5; c++ ))
do
 echo "Number $c"
done

Simple isn't it? If you want an infinite loop: 
#!/bin/bash
for (( ; ; ))
do
   echo "infinite loops [ hit CTRL+C to stop]"
done

If you are working with files: 
#!/bin/bash
for file in /etc/*
do
     echo $file
done

Those are just the basic. There are tons of tutorial online. I will leave the rest of the topics to you.