You've been having that bike for sometime now and it's seen a lot of use, but now, it's started to make clacking noises each time you push the pedals down or worse, you actually feel a slight sideward tremble, as you push the pedals; It's probably time to change the bearings or your entire bottom bracket.  To check, simply park the bike, grip the crank arm on the left side of the bike right at the base (near the pedal) and try to move it sideways, i.e., perpendicular to the direction of its normal arc; there should be no movement. Repeat on the right hand side. If there is any sideways movement where not just the crank arm but the entire crankset (gears) move when you push, it's a sure sign that the bearings or the bottom bracket set needs replacing.  Older bicycles generally have a spindle and two sets of external bearing 'rings', on either side of the cylinder while newer ones have a sealed bottom bracket set, with the ball bearings on the inside.  The spindle itself would almost never merit replacement; a mere replacement of the bearings would suffice.  To take on this job, you would need the following tools.  If you don't have these tools, particularly the crank puller, don't attempt to sledgehammer your way; you'll wreck things. Simply get the tool and then take on the job!

1. Park Tool Universal Crank Puller or equivalent.
2. Park Tool BBT-22 bottom bracket tool or equivalent
3. A sturdy flat-head screwdriver (non-retractable)
4. Hammer
5. Adjustable wrench (45mm clamping width)

Here's what my crank puller equivalent tool from Biltema looks like: Once you take off the dust cap off your crank arm, it exposes the nut (or the screw head). Use the large part of the crank puller on the nut. Simply use it as a normal bolt remover. Once the bolt is off, you can't simply pull off the crank arm, as by design, your pedaling ensures that it is tightened into place. This is where you absolutely need the other side of the crank puller, to get the crank arm off, without any damage. As shown in the pic below, carefully thread the other part of the crank puller into the grooves on inside of the crank arm and tighten the other part of the crank puller into it gradually.  As you tighten it, the bolt pushes against the spindle inside, forcing the crank arm outwards, till it comes away! Huzzah! Perform the same operation on the other side.  My bike had the old style bottom bracket which needs a wrench to open the outer lock-ring.  The new style sealed bottom bracket uses a specialized tool (Park Tool BBT-22 or equivalent) Now's the grunt work, but also the extremely counter-intuitive one ; It loosens CLOCKWISE! That's right! Clockwise! This is to because the clockwise movement when you pedal forward moves it anti-clockwise, which ensures that it stays tight rather than coming off loose. When you get this off, you can simply remove the old bottom bracket set and external bearings if any. Clean out the cylinder of any mud/crud/rust. If it's an old type spindle and external bearing rings, simply swap the old bearings for new, with fresh grease and put the spindle back. If it's a sealed bottom bracket set, simply discard the old and put in the new one and close up, doing everything that you did up to this point, in reverse :) The old style spindle from the bottom bracket The sealed bottom bracket set.  Pic courtesy: www.biltema.se