Ok, buddy. First of all you need a waterproof bag. Remember that.
Second, your bag must have proper compartments where you can store ur stuff without them shaking around when you walk. You know, it must not be like mixed nuts when you open the bag. Organise the stuff properly.
For a basic kit here's what you need:
1. Plasters
2. Antiseptic Lotion.
3. Burn Cream(optional cos you only use it when there is no water source nearby).
4. Triangular bandages(they can double as normal roller bandages too)
5. Gauze dressing.
6. Cotton balls.
7. uhh...how shall I put this...Tape. Not scotch tape but the type u can find in the pharmacy. You use this to secure the bandages.
8. Safety pin. Same purpose as above.
I think I might have left out some stuff...go google "first aid kits" and find out the ideal contents.
