The memory chips you buy for your system are determined by which motherboard you purchased. The motherboard is designed to take a specific type of memory, and that is called out in the motherboard specifications.
Most desktop memory is on 240 pin memory boards, and most motherboards either have two or four 240 pin memory slots. The 240 pin slots are keyed so you can only mount the memory card in one direction.
My recommendation was to maximize your initial memory. That translates to 4 Gb of memory for a 32 bit system. That usually comes on two 2 Gb memory cards. The memory cards are sold together as a pair and are a matched set. The idea is that memory storage is interleaved between the two cards to improve performance. It turns out the slots on the motherboard also are matched to take advantage of the matched memory cards.
On those motherboard that have two 240 pin slots, there isn't a problem as the two cards will plug into the two slcts. Most motherboards have four 240 pin slots, so you need to be careful about which two slots you use for the two memory cards. The motherboard user's guide that comes with the motherboard will tell you how the memory should be installed in the memory slots. Usually, this is slot one and slot three. If you don't find how your slots are set up in the provided user's guide, there is usually much more extensive documentation on the motherboard manufacturer's web site. Once you determine which slots to use, your ready for the install.
Take the memory chip out of the packaging. It should be enclosed in a static free bag. Memory chips are very susceptible to static electricity. Before removing the memory from the bag, ground your self on a metal surface. Handle the memory cards by the card edges.
Place the memory card in the slot in the right direction and push down evenly to sit the card. The card should go in fairly easily and latches on the end should pop up to catch the card and show its seated properly.
That's it. You now have your memory mounted on your motherboard.