The WHO guidelines suggest aiming to get to 6 months on milk alone. However, they are only guidelines. Many babies are ready to move onto simple solids earlier than this, so if you concentrate on introducing babyrice a perhaps a few plain veggies such as carrots, sweet potato, parsnip and fruits such as pear and apple, keeping to that until 6 months, then gradually introduce more variety, arriving at proteins (chicken/fish) being introduced at 7 months, then you wil be up to speed.
It is a bit of an old wives tale that babies watching you eat, or trying to grab at food you are eating means they are ready for solids. It is now considered more likely that this behaviour is simply that your baby has reached an age where they are far more interested in, and interactive with, what is going on around them.
If you had a bouncing 10lb baby at birth, they may well need to be introduced to solids a little earlier than a 7lb baby. It's generally advised that signs are:
Baby wakes in the night (where previously he/she had been sleeping for 10-12 hours straight) and seeming genuinely hungry
Baby finishing full bottle feed but needing more and with more frequency
Baby demanding more feeds from the breast, again with more frequency
Get to as near to 6 months as you can without baby raiding the fridge by night and you will have done well!